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.