• Combine the milk, vanilla extract, and salt in a small bowl, and stir them together to dissolve the salt and set aside.
• Place the butter in a stand mixer fitted with a paddle attachment. Mix for 2 minutes on medium speed, until the butter is lighter in color and creamy. Scrape the bottom and sides of the bowl.
• Add the shortening and the milk mixture, and use a splatter guard over your bowl or place a towel over the top of the mixer to prevent splatters. Run the mixer on low until the liquid starts to incorporate, then turn it to medium until everything is well-mixed and smooth (20-30 seconds). Scrape down the bowl again.
• Keep the towel over the mixer, leaving only a small opening in front, turn the mixer to low, and add the sugar to the bowl in a steady stream. Once all of the sugar is incorporated, scrape down the bottom and sides of the bowl, and turn the mixer to medium-low for 30 seconds to mix the frosting well.
• Divide the frosting equally between 6 bowls. Add a different color of gel food coloring to each bowl and stir well, adding more if necessary until you get a vibrant color. Place each color of frosting into its own piping bag, and snip a 1/2-inch opening in the end of each bag.
• Lay out a piece of plastic wrap about 16 inches long. Start by piping a long line of red food coloring on the plastic wrap. Leave a margin of a few inches from the sides and bottom of the wrap.
• Next, pipe the orange frosting right next to the red, and continue to pipe each color in a line. Depending on how thick you pipe them, you can either fit one line of each color on the wrap, or repeat the pattern and make 12 stripes. Just make sure you leave a margin and don't go all the way to the edge of the plastic wrap. (If you have extra colors left over, you can do this a second time to make another rainbow piping bag.)
• Now, fold the plastic wrap over lengthwise and roll the frosting into a cylinder. Grasp the ends of the plastic wrap in each hand, and twirl it quickly so the frosting is burrito-shaped and the ends of the plastic are tightly twisted.
• Fit a piping bag with a large star tip. Snip off one of the plastic wrap “tails," and insert the frosting cylinder into the piping bag, snipped-side down.
• Squeeze the bag for a moment until all of the colors come out together, then your rainbow piping bag is ready! This recipe makes enough to frost 18 cupcakes with large swirls, or 24-28 cupcakes with small single-layer swirls.

To Make the Cake Layers:

• Preheat the oven to 300 F. (That is not a typo—I use a low and slow bake time for these, to help prevent the cakes from doming.) Line four 9-inch cake pans with parchment rounds, and spray the parchment and pan sides with nonstick cooking spray.
• Combine the sugar, cake flour, baking powder, baking soda, and salt in the bowl of the stand mixer fitted with a paddle attachment. Mix on low speed for about a minute to combine the dry ingredients.
• Lightly whisk together the eggs and 1/3 cup of sour cream, and set aside for now.
• Add the remaining 1 cup sour cream and the softened butter to the large mixing bowl and mix on low speed until the dry ingredients are moistened. Turn the mixer to medium speed and mix for a minute and a half.
• Add the egg mixture in three batches, mixing for 20-30 seconds after each addition, until you have a smooth batter. Scrape the bottom and sides of the bowl with a rubber spatula, and give the batter a few more stirs to make sure everything is incorporated. Divide the batter into four equal portions—if you are using a kitchen scale, this will be about 15 oz of batter per bowl.
• To one portion of the batter, add the zest from one large lemon, 1 tsp lemon extract, and a few drops of yellow food coloring. Taste, and add more extract if desired, then scrape into one of the cake pans and smooth.
• To the second portion of the batter, add the zest from two large limes, 1 tsp lime extract, and a few drops of green food coloring, with an additional drop or two of yellow. Scrape into the second cake pan.
• Place the freeze-dried raspberries in a food processor and process until they’re a fine powder. Add the raspberry powder and some pink food coloring to the third portion of batter, and gently stir until it’s incorporated. Try not to over-mix, or your cakes will be tough. Pour into the third cake pan.
• Process the freeze-dried blueberries in the food processor, and add the blueberry powder and purple coloring to the final portion of batter. Pour into the fourth cake pan.
• Bake the cakes at 300 F for 20-25 minutes, rotating halfway throughout, until the tops spring back when lightly pressed and a toothpick inserted into the center comes out clean.
• Let the cakes cool on a wire rack for 10 minutes, then gently turn them out and let them cool completely. If you are making these in advance, they can stay in the freezer for up to several weeks.

To Assemble:

• Prepare your pan. You need a 9-inch pan with tall sides, and the ability to remove the bottom. Either a cake pan with a removable bottom or a springform pan will work. Place a cake cardboard in the bottom of the pan. Tape two long strips of an acetate cake collar together, to make one 8-inch tall strip of acetate. Line the inner wall with your extra-tall acetate collar. Make sure the acetate is long enough so that it overlaps itself. I strongly recommend using acetate—it will provide you with a firm structure in which to build your cake, and clean edges along the sides. If you don’t have acetate collars, you can try using a double layer of heavy-duty foil, 8 inches tall, to wrap around the inside of the cake pan. This should be stable enough, but it won’t produce the clean edges that acetate does.
• Set your lemon cake layer in the bottom of the cake pan on the cardboard. Prepare your passion fruit mousse (instructions below), pour it on top of the lemon cake, and smooth it into an even layer. You’ll notice that the mousse is somewhat loose and fluid, which is why it’s so important to have a stable layer of acetate or heavy duty foil around the sides of the cake, to prevent the mousse from overflowing the sides.
• Chill the cake for 10 minutes, so the mousse just starts to set but is still tacky to the touch. Add the raspberry cake layer on top, and return to the refrigerator while you prepare the cranberry mousse layer (instructions below).
• When the cranberry mousse is done, pour it on top of the raspberry cake, and spread it into an even layer. Chill the cake again for 10 minutes, then add the blueberry cake on top. Refrigerate again while preparing the final mousse, the blackberry layer (instructions below.)
• When the blackberry layer is done, pour it on top of the blueberry cake and smooth it. Chill for 10 minutes, then top with the final lime cake layer. Refrigerate the cake for at least 6 hours (longer is okay!) to firm up all the mousse and make it a stable, solid cake.
• To serve, remove the cake from the cake pan, then carefully unwrap the acetate or foil collar from around the cake. Top with vanilla frosting (recipe below) or whipped cream, and lots of sprinkles and candles! This cake is easiest to cut when it’s straight from the refrigerator, but I like it best when it’s sat at room temperature for 20-30 minutes, so the cake is not completely cold.

To Make the Passion Fruit Mousse:

• Combine the white chocolate, passion fruit puree, 1/4 cup of cream, and salt in a large microwave-safe bowl. Microwave in 30-second increments, stirring every 30 seconds, until the chocolate has melted and the mixture is smooth. Let it cool to room temperature, stirring occasionally. It should not be warm to the touch at all.
• While you wait for the white chocolate mixture to cool, prepare the gelatin. Whisk together the gelatin and the cold water in a small bowl, and set it aside to let the gelatin absorb the water. When the chocolate is at room temperature, microwave the bowl of gelatin for 15 seconds, until it is melted. Whisk the melted gelatin and white chocolate together. Add several drops of orange food coloring, and stir them in. The color will get very diluted when you add the whipped cream, so go darker than your intended color.
• Whip the remaining 1 cup heavy cream to firm peaks. Fold half of the whipped cream into the white chocolate, and once it’s incorporated, gently fold in the remaining whipped cream. Try not to fold more than necessary, or the mousse will start to get grainy and break. Use immediately after it’s made.

To Make the Cranberry Mousse:

• Follow the instructions for passion fruit mousse, but with a few small changes: combine the white chocolate, 1/2 cup cream, and salt and microwave together. Use red food coloring instead of orange, and add enough to make a very bright red color. Place the freeze-dried raspberries in a food processor and process until they’re a fine powder. After you fold in the whipped cream, add the cranberry powder and stir it in gently.

To Make the Blackberry Mousse:

• First, blend the blackberries in a blender or food processor, then strain them to remove the seeds. Measure out 1/2 cup puree (save any additional puree for another use) and combine 1/2 cup puree, the white chocolate, and the salt in a microwave-safe bowl. Follow the instructions for the passion fruit mousse, but use blue food coloring instead of orange.

To Make the Vanilla Frosting:

• Place all of the ingredients in the bowl of a large stand mixer fitted with a paddle attachment. Mix on low speed until the sugar is moistened. Scrape down the sides of the bowl, then beat again on medium speed until light and fluffy, 2-3 minutes. You can add more milk or sugar, if desired, to get the consistency you want.

To Make the Cake:

• Preheat the oven to 350 F. Line a 13×18″ rimmed baking sheet with parchment, and spray the pan and parchment well with nonstick cooking spray. In a small bowl, whisk together the flour, baking powder, and salt, and set aside.
• Place the lemon zest and the sugar in the bowl of a large stand mixer, and rub them together between your fingers until the sugar is fragrant and has the texture of wet sand. Add the butter and cream cheese to the bowl. Fit the mixer with a paddle attachment, and mix on low speed until combined, then raise the speed to medium and beat until light and fluffy, about 3-4 minutes.
• Add the eggs one at a time, beating well after each addition, then add the lemon extract.
• Finally, turn the mixer to low, and slowly add the dry ingredients. Stop when just a few streaks of flour remain. Finish mixing the cake by hand, using a spatula to scrape the bottom and sides of the bowl well.
• Coarsely crush the freeze-dried raspberries (using either a knife or your hands is fine) and add them to the batter, stirring them in until well-mixed.
• Scrape the cake batter out into the prepared pan and spread it into an even layer. Bake the cake for about 20 minutes, until it is a light golden color, the top springs back when lightly pressed, and a toothpick inserted into the center comes out clean. Cool completely on a wire rack. The cake can be made several days in advance and kept, well-wrapped, at room temperature or in the freezer. If freezing, defrost partially before using.

To Make the Frosting:

• Combine the egg whites, granulated sugar, and salt in the bowl of a large stand mixer, and whisk them together. Choose a small saucepan that lets you fit the base of the stand mixer snugly into the top of the saucepan—this is your makeshift hot water bath. (Alternately, you can use a different bowl or an actual bain marie and then transfer the mixture to a mixing bowl once it’s heated.) Add an inch of water to the bottom of the saucepan, and bring the water to a simmer.
• Place the mixing bowl on top of the saucepan, making sure that the bottom isn’t in contact with the water, and heat the egg white mixture. Whisk frequently so that the egg whites don’t cook. Continue to heat the whites until they are hot to the touch, and when you rub a bit between your fingers, you don’t feel any grittiness from the sugar.
• Once the whites are hot, transfer the mixing bowl to your mixer and fit it with a whisk attachment.
• Beat the whites on medium-high speed until they are no longer warm to the touch—feel the outside of the bowl, and make sure that it is around room temperature. Depending on your mixer and the temperature of your environment, this may take 10-20 minutes, or more.
• Reduce the speed to medium-low and add the softened but cool butter in small chunks, a tablespoon at a time, making sure to wait in between additions. It may separate or look a little gloopy at this point—fear not. Once all of the butter is added, increase the speed again and whip until it comes together and is light and fluffy. If, after 5 minutes, it hasn’t come together, refrigerate the mixing bowl for 5-7 minutes, to cool the mixture down, and whip it again.
• Once the butter is fluffy, add the vanilla extract and mix until well-blended. Separate out about two-thirds of the buttercream, and to the remaining third, add the pistachio paste and a drop or two of green food coloring, and mix until well-blended.
• The buttercream can be made in advance and kept at room temperature if you’re going to use it the same day, or refrigerated. If it’s been chilled, let it sit at room temperature until it softens, then re-whip it to get the fluffy texture back before you use it.

To Assemble:

• Cut the cake in half widthwise, so you have two rectangles that are about 7”x9”. Place one half on a cake board, and spread the pistachio buttercream on top of the cake, then place the second cake layer on top.
• Cut the large cake into four smaller cakes – squares are easiest, but you can also experiment with circles or rectangles if you want a variety. Cut cake boards to size, and put one under each mini cake.
• Run a spatula around the sides of the cakes to smooth out any pistachio frosting that is sticking out. Spread a very thin layer of vanilla buttercream on the top and sides of the cakes, and refrigerate until the frosting is firm, about 30 minutes.
• Spread the remaining vanilla buttercream over the top and sides of the cakes. Use an angled spatula or bench scraper to get the sides even and the corners neat. (Don’t worry if they're not perfect—you can cover up any bad spots with polka dots or stripes!) Refrigerate the cake again to firm up the frosting.
• Cut out circles, long ribbons, or other shapes from sheets of Fruit Roll-Ups. Press them into the sides of the cake. If you need the Fruit Roll-Ups to stick to themselves, dab them with a small amount of water. The Fruit Roll-Ups will start to get sticky after sitting on the frosting for a few hours, so this decoration is best added right before the cakes will be eaten.
• For the best taste and texture, enjoy these cakes at room temperature.

To make the cake:

• Preheat the oven to 350 F. Line four 6-inch cake pans with parchment circles and spray them with nonstick cooking spray, or brush them with pan release.
• Add the butter and both sugars to the bowl of a large stand mixer fitted with a paddle attachment. Beat them together on medium-high speed, until light and fluffy, about 5 minutes. Add the eggs one at a time, beating well after each addition. Add the vanilla extract and espresso powder, if using, and mix them in.
• In a different bowl, sift together the flour, cocoa powder, baking soda, and salt. With the mixer running on low, add a quarter of the sifted flour mixture. When the flour streaks have almost all disappeared, add a third of the liquid to the mixing bowl. When that’s incorporated, continue to add the drys and wets in an alternating pattern, ending with the dry ingredients.
• When the dry ingredients are nearly incorporated, stop the mixer. Scrape down the bottom and sides of the bowl with a rubber spatula, and finish mixing the cake by hand.
• Divide the batter between the prepared pans — each should get about 16 oz batter. Bake the cakes at 350 F for about 45 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Cool the cakes for 15 minutes, then turn them out of the pans and cool completely.
• The cakes can be made several days in advance and kept, well-wrapped and frozen, until ready to use. If you freeze them, make sure they are partially defrosted before using them so they can be cut easily.

To make the black buttercream:

• Place the butter in the bowl of a stand mixer fitted with a paddle attachment. Beat the butter for 2-3 minutes, until light and creamy.
• While the butter is beating, mix together 4 TBPS of milk, the vanilla, and the salt in a small bowl, and stir to help the salt dissolve.
• With the mixer on low, add the milk mixture. At first it will resist coming together, but raise the speed to medium-low and continue to beat it for about a minute, until it comes together and you have a smooth and creamy mixture.
• Sift both cocoa powders together to remove any lumps.
• Turn the mixer back to low and add the cocoa powder. When it’s mixed in, add about half of the powdered sugar.
• Let that powdered sugar incorporate, then add the rest of the powdered sugar. Let it mix, then stop the mixer and scrape down the bottom and sides of the bowl to get any dry pockets.
• Mix at medium speed for 2 minutes, then lower it to the lowest speed and run the mixer for about 5 minutes. If the frosting is very stiff, add additional milk, a teaspoon at a time, until it is creamy, smooth, and easily spreadable.
• Add the black food coloring and mix it in. Add additional coloring if desired. The black color will darken and deepen over time, so if you’re able to make this a day in advance, you’ll have more success getting a true dark black.
• Press a layer of plastic wrap directly on top of the frosting, and store it at room temperature for up to a day, or in the refrigerator for a week, until ready to use. When you’re going to use it, stir it for several minutes by hand with a rubber spatula to remove any air bubbles and restore its creamy texture.

To make the colored buttercream:

• Place the butter in the bowl of a stand mixer fitted with a paddle attachment. Beat the butter for 2-3 minutes, until light and creamy.
• While the butter is beating, mix together the milk, vanilla, and salt in a small bowl, and stir to help the salt dissolve.
• With the mixer on low, add the milk mixture. At first it will resist coming together, but raise the speed to medium-low and continue to beat it for about a minute, until it comes together and you have a smooth and creamy mixture.
• Turn the mixer back to low and add about half of the powdered sugar.
• When that’s mixed in, add the rest of the powdered sugar. Let it mix, then stop the mixer and scrape down the bottom and sides of the bowl to get any dry pockets.
• Mix at medium speed for 2 minute, then lower it to the lowest speed and run the mixer for about 5 minutes. At the end, your frosting should be creamy and smooth.
• Divide the frosting evenly into 6 bowls–each should have a little over a cup of frosting. Add a big squirt of neon food coloring to each bowl and stir well, then add additional coloring if you want a brighter color. All food coloring intensifies over time, so if you’re able to make this several hours or even a day ahead of time, you’ll have brighter colors.
• Press a piece of cling wrap directly on top of the frosting and keep it covered until you’re ready to use it, so it won’t dry out. When you’re ready, transfer each color into a piping bag fitted with a small round #4 tip.

To assemble:

• If your cakes have domed, trim off the tops to make them flat, then slice each cake in half, so that you have eight thin cake layers.
• Leave three of the layers as they are, and for the remaining five, use a 3-inch circle cutter to cut a circle out of the center of the cakes. You can eat the centers, or save for another use. Put one of your intact cake layers on a cake cardboard to be the base of the cake.
• If you are using acrylic discs, put a nonskid pad on your cake turntable, then a 6.5” disc, then another nonskid pad, then top with a cake on a cake cardboard. If you aen’t using discs, just place the cake on a cardboard on your cake turntable with a nonskid pad underneath if you have one.
• Spread about ½ cup of black frosting on top of the base cake layer. Take a second cake layer without the center hole, and place it on top. Spread more frosting on this second layer.
• Now grab those layers with the center holes! Stack them, one on top of the other, with a thin layer of buttercream between each. I like to use a piping bag to pipe a ring of buttercream on the cake rounds, and then spread it evenly with a mini spatula. Try to make sure you’re stacking the cake as straight as possible as you add the layers.
• Take a mini spatula and spread a thin layer of frosting on the walls of the hole inside the cake– this just helps the cake’s stability and also seals in the cake so it stays moist. Refrigerate the cake for 20-30 minutes, until the frosting is firm.
• Fill the hole in the cake with candy of your choice. I recommend candy-covered chocolates like Sixlets or M&Ms, or candies that are individually wrapped. Avoid unwrapped hard candies–they will absorb too much moisture from the cake and will be a sticky mess.
• Add a thin layer of frosting around the top of the cake, and press the final cake layer on top. Spread a thin layer of frosting all over the top and sides of the cake to lock in any crumbs. Refrigerate for at least an hour, until firm.
• To finish frosting the cake, spread a thick layer of frosting on top of the cake and smooth it out all the way to the sides. If you are using acrylic discs: take a second 6.5” disc and place a few dabs of frosting along the outside edge. Press a parchment circle onto the disc, then place the disc parchment side-down on top of the cake. Try to make sure the top disc is in line with the bottom disc.
• Cover the sides of the cake with frosting. Take a tall straight edge and start scraping the extra frosting off, filling in the areas where it’s sparse. As you work, make sure you are holding the scraper in a straight line against both the top and bottom disc. Fill in any frosting gaps, and keep filling and scraping until you’re happy with the buttercream. Chill for at least 20 minutes, then run a hot, thin sharp knife under the top disc so that it pops off. Carefully peel off the parchment from the top.
• If you are not using acrylic discs: Once the top is frosted, spread a thick layer of black frosting along the sides of the cake. Use a metal spatula, bench scraper, or tall straight edge to smooth the frosting into an even layer. Wash the spatula frequently in hot water to make the smoothing process easier.
• Scoop a little more black frosting on top, and spread it almost to the edge. Take a mini spatula and hold it against the cake as you turn it on the turntable, to create a spiral design. Don’t worry about making the cake edges look nice, since those will be covered up later. Chill the cake for at least an hour, until very firm. This is also a good stopping place if you want to refrigerate it overnight.
• To decorate:
• Use a pastry brush to brush the bottom inch of the cake with corn syrup or piping gel, and press sprinkles all around to make a sprinkle border.
• Cut the pinata donkey or other shape of your choice out of cardstock. With one hand hold it against the cake, and with the other lightly trace around it with a toothpick.
• Take one of your frosting bags, and use an up and down motion to make a small zig-zag pattern across the bottom of the shape.
• Once you’ve done your first line, use a different color for the second layer. Make sure you slightly overlap the first layer so that it looks like the second color is laying on top of the first. Keep repeating this design with different colors as you work your way up the pinata shape.
• When the shape is completely filled in, swap the small round tip for a large French star tip for each piping bag. (I used Wilton #8B). Pipe a rope border in alternating colors all along the top edge.
• It’s easiest to cut this cake when it is still chilled, but not straight from the refrigerator. (You want the stability of the cold to keep the slices intact, but straight from the fridge can make the frosting extremely hard.) The candy will roll everywhere once it’s cut, so put it on a rimmed cake stand or plate if you want to avoid that!
• Cut the cake into slices (don’t forget to ooh and aah as the candy pours out) and serve with a few pieces of candy on the side! For the best taste and texture, enjoy this cake at room temperature. Store leftovers, covered, at room temperature for 3 days or in the refrigerator for up to a week.

To Make the Cakes:

• Preheat the oven to 350 F. Line two 5-inch cake pans and two 8-inch cake pans with parchment rounds, and spray the parchment and pan sides with nonstick cooking spray. (Note that you can make these cakes in different sizes depending on what size pans you have. If you change the pan size, just watch them closely as the baking time will most likely change as well.)
• Combine the sugar, cake flour, baking powder, and salt in the bowl of the stand mixer fitted with a paddle attachment. Mix on low speed for about a minute to combine the dry ingredients.
• Lightly whisk together the egg whites together with a quarter of the milk and the vanilla extract, and set aside for now.
• Add the remaining milk and the softened butter to the large mixing bowl and mix on low speed until the dry ingredients are moistened. Turn the mixer to medium speed and mix for a minute and a half.
• Add the egg white mixture in three batches, mixing for 20-30 seconds after each addition, until you have a smooth batter. Scrape the bottom and sides of the bowl with a rubber spatula, and give the batter a few more stirs to make sure everything is incorporated. Add the rainbow nonpareil sprinkles, and stir just a few times to mix them in—don’t over-mix, or the batter will be streaky.
• Divide the batter between the pans, filling each about half full. The 5” pans should get about 9 oz of batter, and each 8” pan should get about 18 oz of batter. Bake the cakes at 350 F for 30-35 minutes, until the sides just start pulling away from the pan, and a toothpick inserted into the center comes out clean.
• Let the cakes cool on a wire rack for 10 minutes, then gently turn them out and let them cool completely. I find it’s easiest to cut and stack cakes that have been chilled, so if you have time, wrap them with plastic wrap and put them in the freezer for at least 30 minutes, until they are firm but not frozen all the way through. If you are making these in advance, they can stay in the freezer for up to several weeks. Let them partially defrost, until they are firm but not rock-hard, before assembling.

To Make the Buttercream:

• Combine the whites, powdered sugar, and salt in the bowl of a large stand mixer fitted with a paddle attachment. Mix everything together on low speed, until the sugar is moistened and no dry patches remain. Turn off the mixer, scrape down the sides and bottom of the bowl with a spatula, then turn the mixer to medium speed. Beat on medium for 5 minutes.
• After 5 minutes, turn the mixer to medium-low and start adding the softened room temperature butter, 1-2 tablespoons at a time. Once all of the butter is incorporated, add the vanilla extract and mix it in. Stop the mixer and scrape down the bottom and sides once more. Turn the speed to medium and beat the buttercream for 10 minutes. At the end, you’ll have a frosting with a wonderfully light and creamy texture.
• Separate out about three-quarters of the frosting, and add enough pink gel food coloring to get a bright, vibrant pink that matches the color of your animal cookies. (I used Americolor Electric Pink, and found that the brand of cookies I bought were more “hot pink” than “soft pink.” Your milage may vary!) Keep the remaining quarter of the frosting white.

To Assemble:

• Slice each cake round in half so you have four rounds of each size. Place an 8-inch cake on a cake cardboard and place it on a cake turntable. Place a big dollop of pink frosting on the cake layer—about 2/3-3/4 cup—and use a metal spatula to spread the frosting into a layer going all the way out to the edges of the cake.
• Sprinkle a handful of the finely chopped cookies on top of the frosting, leaving a margin around the edge, and press gently to embed them. Add a little more frosting—about ¼ cup—on top of the cookies, and spread it out on top.
• Top the cake round with a second cake layer, and repeat the frosting and cookie process again. Do it with a third layer, and finally, add the last cake layer. Cover the top and sides of the cake with a very thin layer of pink frosting—this is the crumb coat, which locks crumbs into the frosting and also provides some stability to the cake. Refrigerate the cake for 30-45 minutes, until the crumb coat is set and the cake feels firm and stable.
• Cover the top and sides of the chilled cake with a thicker layer of pink frosting. Use a bench scraper or metal spatula to smooth out the sides and top of the cake as much as possible. Chill for 30-45 minutes, until firm.
• Pour the rainbow sprinkles into a shallow bowl, and supporting the cake in one hand, use the other hand to pick up a handful of sprinkles and press them against the bottom of the cake. Go around the entire bottom edge of the cake adding a border of rainbow sprinkles. Even though the frosting is chilled, the moisture from the frosting should be enough to cause the sprinkles to stick. If they’re falling off, let it soften at room temperature for a bit and then try again.
• For the top tier of the cake, repeat the slicing and filling process with the 5-inch cake layers. This time, use the white frosting for the crumb coat and final coating on the cake, and refrigerate the assembled cake for at least an hour, until very firm.. When you’re ready to apply the drip, set a piece of parchment on top of the cake, top it with a cardboard round, and flip it over so the bottom is now the top.
• Prepare the ganache drip by combining the candy melts and cream in a small bowl, and microwave for 45 seconds. Whisk well, and if any chunks of candy coating remain, whisk again briefly until it is completely melted and smooth.
• Transfer the glaze to a squeeze bottle with a narrow tip. Squeeze a dime-sized amount of glaze along the top edge (which is actually the bottom!) of the chilled cake, and nudge it gently so it starts to slide down the side. Repeat right next to the first drip, and continue around all the edges of the cake. Refrigerate the cake round for 15-20 minutes, until the glaze is completely set.
• Place bottom tier of the cake on your serving plate or cake stand. Insert 4 cake dowels in a ring in the center of the cake, and trim them so they’re flush with the top of the cake. Add a dab of buttercream on top to adhere the top layer, then add the second cake on top.
• To finish the cake, you can top it with an oversized sugar cookie made to look like a circus animal cookie, or to simplify things, just add a few buttercream rosettes and top with regular frosted animal cookies.
• For the best taste and texture, serve this cake at room temperature. Leftovers can be stored in an airtight container in the refrigerator for up to a week, but will taste best when eaten within 3-4 days.

To Make the Agate Candy Slices:

• Take a square of nonstick heavy-duty foil, nonstick side up. Place a small bowl or cup on top, to serve as a guide, and fold up the edges to form a circular mold with at least a 1-inch lip around the center. Repeat, making different shapes and sizes, for as many candy agate slices as you want to make. Place the foil molds on a baking sheet, and preheat the oven to 350 F.
• Divide the candies by color. While they are still in their wrappers, use a meat mallet, rolling pin, or other large, heavy object to crush them into small pieces—the closer you can get to candy dust, the better! Undo the wrapper and pour the crushed candies into small bowls, divided by type of candy. The more varieties of color and opacity you can get, the better! After the candy is crushed, it will start to get sticky as it is exposed to the humidity in the air, so start to form the agate slices immediately.
• Place a few spoonfuls of one color into a foil mold, and use a small food-safe paintbrush to spread it out along the outer edges of the mold in an even layer. (You can use your fingers, but the candy tends to stick to skin, so a paintbrush is faster and less messy.) Make sure that the layer is not too thin—as the candy melts, it will thin out a bit, so make the layer at least ¼-inch tall if possible. Add a second, different color, and brush that one out to the edges next to the first. Continue to make concentric rings of candy pieces in the mold, alternating colors and opacities, until you’ve filled the whole mold with crushed candy. I found I liked it best if I worked in one color scheme (shades of blue, for instance, or warm-toned colors) for each agate slice, but you can experiment to see what works for you.
• Once all of the foil molds are filled, bake them in the oven at 350 F for 4-5 minutes. Check them after 4 minutes to see if all the candy pieces have melted. If some unmelted pieces remain, bake for another 30-60 seconds—just until it’s melted but the bubbling is minimal. Let the candy cool completely, then gently peel back the foil mold.
• If desired, mix some edible silver luster dust with a bit of vodka to make a colored paste, and paint a thin silver line around the edges of the agate slices. Store the slices in an airtight container until you’re ready to decorate the cake.

To Make the Chocolate Cake:

• Line four 8-inch cake pans with parchment, and spray them with nonstick cooking spray. Preheat the oven to 350 F.
• Add the butter and both sugars to the bowl of a large stand mixer fitted with a paddle attachment. Beat them together on medium-high speed, until light and fluffy, about 5 minutes. Add the eggs one at a time, beating well after each addition. Add the vanilla extract and mix it in.
• In a separate bowl, whisk together the buttermilk, sour cream, and water (or coffee, if using). In a different bowl, sift together the flour, cocoa powder, baking soda, and salt. With the mixer running on low, add a quarter of the sifted flour mixture. When the flour streaks have almost all disappeared, add a third of the liquid to the mixing bowl. When that’s incorporated, continue to add the drys and wets in an alternating pattern, ending with the dry ingredients.
• When the dry ingredients are nearly incorporated, stop the mixer. Scrape down the bottom and sides of the bowl with a rubber spatula, and finish mixing the cake by hand. Divide the batter between the prepared pans, giving each one about 11 ounces, and smooth it into an even layer. Bake the cakes for 22-25 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Cool the cakes completely before assembling.

To Make the Buttercream:

• Combine the whites, powdered sugar, and salt in the bowl of a large stand mixer fitted with a paddle attachment. Mix everything together on low speed, until the sugar is moistened and no dry patches remain. Turn off the mixer, scrape down the sides and bottom of the bowl with a spatula, then turn the mixer to medium speed. Beat on medium for 5 minutes.
• After 5 minutes, turn the mixer to medium-low and start adding the softened room temperature butter, 1-2 tablespoons at a time. Once all of the butter is incorporated, add the vanilla extract and mix it in. Stop the mixer and scrape down the bottom and sides once more. Turn the speed to medium and beat the buttercream for 10 minutes. At the end, you’ll have a frosting with a wonderfully light and creamy texture. It can be used right away, or stored in the refrigerator for up to two weeks, or the freezer for up to two months. If you have chilled the frosting, let it come to room temperature and then beat it for several minutes to smooth it out and restore its texture before using it.

To Assemble:

• Decide what colors you want to use for your cake—I used blue, purple, and pink. Reserve 1 cup of white frosting, tint ½ cup of frosting black, then use gel food coloring to make a dark and light tone for each color you’re going to use (I made dark and light blue, dark and light purple, and dark and light pink.) Place each color in its own piping bag.
• Put a cake layer on a cake cardboard, and place it on a cake turntable. Pipe concentric rings of frosting, inspired by the dark and light bands of agate slices. My first frosting layer had a thin line of black frosting along the outside, then dark blue, then white, then light blue, then black, then a white center. You don’t have to do this exactly, just alternate colors in the same shade family to make concentric rings.
• Add a second cake layer on top, and do the same thing with a different color family: I used shades of red and pink for this layer. Add a third layer, and this time, use another set of colors (I used purple shades). Finally, top with your fourth cake layer. Spread a thin layer of frosting all along the top and sides of the cake, to lock in any crumbs, and refrigerate for 30 minutes until firm.
• Take the piping bags of colored frosting, and pipe dots and small lines of different colors of frosting all along the sides of the cake, until it’s nearly covered with a random assortment of different colors. Take a metal spatula or bench scraper and hold it against the side of the cake while slowly turning the cake on the turn-table. It will smooth out the frosting and cause the colors to blur together in a watercolor effect. Wipe off the excess frosting from the scraper, and repeat until the sides are smooth and you are happy with the design. If there are places that look too light or too dark, pipe a few dots of another color there and go back with the bench scraper to smooth it out.
• Do the same thing with the top of the cake, piping different colored dots and lines, then smooth it out with an offset spatula. Go along the edges and remove any excess buttercream from the edges of the cake. If you want to touch up the design, refrigerate the cake for 30-45 minutes, until the buttercream is hard, then go back in with a small spatula and blend in different colors onto the cake—it’s easier to get a nice blend if you’re working with a firm base layer of buttercream.
• Finally, press skewers or long toothpicks into the top of the cake so they stick out an inch or two, and use those to support the agate slices. Arrange the candy slices on top in an overlapping pattern. Serve at room temperature, and enjoy!

To Make the Fondant Decorations:

• The horns and ears will need at least a day to harden, so you’ll want to do these first.
• Dust your work surface with powdered sugar. Pinch off a golf ball-sized piece of fondant, and roll it out into a long, skinny snake. Pull the two ends of the snake together, and begin twisting the fondant rope together until it forms a tight coil, at least 6-7 inches long. Twist the top so that it thins out into a twirly point. Cut off the excess at the bottom, and transfer your first unicorn horn to a baking sheet covered with parchment. Repeat two more times until you have 3 unicorn horns.
• Roll out the remaining fondant into a thin layer about ¼-inch thick. Use a circle cutter, cut overlapping circles so that you form leaf shapes—these will be the unicorn ears. Make 6 of these, and put them on the baking sheet as well.
• If you have pearlescent food spray, spray the horns and ears now.
• Use fondant tools or the dull tip of a table knife to score along the outer edge of each ear, leaving about a ¼-inch margin around the edge. Paint the inside with a light coating of corn syrup, and place the colorful sprinkles of your choice inside the ear, pressing gently to adhere them to the fondant.
• Once all of the ears are decorated, leave the fondant in a dry place to firm up, for at least a day. (If your environment is humid, it might take more than one day, so it’s best to do these well in advance of when you need them.)

To Make the Cakes:

• Preheat the oven to 350 F. Line six 5-inch cake pans with parchment rounds, and spray the parchment and pan sides with nonstick cooking spray. (Note that you can make these cakes in different sizes depending on what size pans you have. If you make larger cakes, just extend the baking time and watch them closely. You can also bake them in batches if you only have a few 5-inch cake pans.)
• Combine the sugar, cake flour, baking powder, and salt in the bowl of the stand mixer fitted with a paddle attachment. Mix on low speed for about a minute to combine the dry ingredients.
• Lightly whisk together the egg whites together with a quarter of the milk and the vanilla extract, and set aside for now.
• Add the remaining milk and the softened butter to the large mixing bowl and mix on low speed until the dry ingredients are moistened. Turn the mixer to medium speed and mix for a minute and a half.
• Add the egg white mixture in three batches, mixing for 20-30 seconds after each addition, until you have a smooth batter. Scrape the bottom and sides of the bowl with a rubber spatula, and give the batter a few more stirs to make sure everything is incorporated. Divide the batter into 6 even portions, and color them a rainbow of colors: I used yellow, orange, pink, purple, green, and blue.
• Use a spoon to scoop spoonfuls of the batters and plop them in a random pattern in the prepared cake pans. Fill them about halfway full—each pan should have about 9 oz of batter. Swirl a knife through the batter gently to mix up the colors. Bake the cakes at 350 F for 30-35 minutes, until the sides just start pulling away from the pan, and a toothpick inserted into the center comes out clean.
• Let the cakes cool on a wire rack for 10 minutes, then gently turn them out and let them cool completely. I find it’s easiest to cut and stack cakes that have been chilled, so if you have time, wrap them with plastic wrap and put them in the freezer for at least 30 minutes, until they are firm but not frozen all the way through. If you are making these in advance, they can stay in the freezer for up to several weeks. Let them partially defrost, until they are firm but not rock-hard, before assembling.

To Make the Buttercream:

• Combine the whites, powdered sugar, and salt in the bowl of a large stand mixer fitted with a paddle attachment. Mix everything together on low speed, until the sugar is moistened and no dry patches remain. Turn off the mixer, scrape down the sides and bottom of the bowl with a spatula, then turn the mixer to medium speed. Beat on medium for 5 minutes.
• After 5 minutes, turn the mixer to medium-low and start adding the softened room temperature butter, 1-2 tablespoons at a time. Once all of the butter is incorporated, add the vanilla extract and mix it in. Stop the mixer and scrape down the bottom and sides once more. Turn the speed to medium and beat the buttercream for 10 minutes. At the end, you’ll have a frosting with a wonderfully light and creamy texture.

To Assemble and Decorate:

• Place the cakes on small cake cardboard rounds, and set one on a cake turntable. Use a sharp serrated knife to cut off the domed top, if necessary, and to cut each cake into 2 layers.
• Spread a layer of white buttercream on the first cake round, and top with a second cake layer. Repeat until your first cake has 4 layers of frosting, and cover the top and sides with a very thin layer of buttercream. Repeat with the remaining cake layers—you should end up with 3 cakes, each with 4 layers. Refrigerate the cakes for 20-30 minutes to firm up.
• Fit a piping bag with a large star tip, and fill it with about 1 ½ cups of white buttercream. Set aside for now, and divide the remaining buttercream into smaller bowls. Color the divided frosting into the same rainbow colors you used for the cake batter.
• When the cakes are firm, place one back on the cake turntable. Use a spatula to smear colors of frosting in a random pattern all over the cake. Don’t worry about it looking “pretty”—at this point, you just want to cover the cake with colorful dabs of frosting. Once the top and sides of the cake are colored, use a bench scraper and slowly drag it along the outside of the cake, removing excess frosting and smearing the colors together for a painterly effect. It might take several passes with the bench scraper to smooth out the frosting.
• Use a mini offset spatula on top of the cake, and holding it in the center, turn the cake on the turntable to smooth out the top. Finally, scrape the spatula in from the cake edges to remove any excess frosting from the corners. Don’t worry about it being perfect, because most of the top will be covered with decorations, and besides, who wants a perfect unicorn? Repeat with the remaining 2 cakes.
• Take the white frosting with the star tip, and pipe a swirled star on the front of each cake, for the unicorn’s mane. Pipe stars and swirls on top for the mane, and decorate the mane with colorful sprinkles and sugar pearls. Switch out the star tip for a small round tip (I used a #3) and give each unicorn eyes.
• Finally, take your fondant ears and horn and press them firmly into the top, embedding them in the cake. Your unicorns are finished! For the best taste and texture, serve these cakes at room temperature.