Separate out 5 portions of frosting, each about 3/4 -1 cup, into individual bowls. Color the frosting with gel food coloring, until you have a vibrant yellow, green, pink, purple, and turquoise. Fit a piping bag with a large round tip and fill it with white frosting.
Slice each cake round in half so you have six rounds total. Place an 8-inch cake on a cake cardboard and place it on a cake turntable. Pipe a layer of white frosting around the outer edge of the cake—this acts like a barrier to prevent the colored frosting from seeping through.
Place a big dollop of yellow frosting on the cake layer—about a generous ½ cup—and use a metal spatula to spread the frosting into a layer going all the way out to the edge so it meets the white buttercream border.
Top the cake round with a second cake layer, and repeat the frosting process again, this time with a different interior color. Continue to layer the cake this way, until you’ve used all 5 colors and 6 cake rounds.
Cover the top and sides of the cake with a very thin layer of white 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 white frosting. Aim for this layer to be about ½-inch thick. This will seem like WAY too much frosting, but trust me! Most of it will get scraped off during the process, but it’s easier to start with a thicker layer. Use an icing smoother to smooth out the sides and top of the cake as much as possible. It’s not important that it be perfect.
Next, use an icing comb to make ridges on the cake. Hold the comb against the side of the cake, straight up and down but angled back close to the side of the cake, rather than perpendicular to the side. Slowly turn the cake as you scrape off the first layer of frosting.
The first one or two times you use the comb, the stripes will look very patchy and rough. This is normal! After every pass, wipe or wash off the comb so it’s clean, then fit the comb into the grooves and turn it around the cake again. After a few more turns, your stripes should be looking pretty good! If you uncover bare cake areas during this process, you can always add more frosting with a spatula and then go around again with the icing comb.
Once your ridges are made, freeze the cake for 30 minutes, or refrigerate for at least 60, until the cake is VERY firm.
Transfer your remaining colored buttercream into individual piping bags, and snip off the tip to make a medium-sized opening. Pipe one color into the hollow part between each buttercream stripe on your cake. It’s fine if the lines aren’t perfect or if they go over onto the white portion. Repeat until you’ve filled every ridge with colored buttercream.
Use the straight-sided scraper to go around the sides of the cake, smoothing it out. At first, it will smear the colors over the sides, but keep turning (and cleaning the scraper between turns!) Gradually you will remove the excess colored buttercream, and reveal the beautiful, clean white stripes underneath. Since the cake was very well chilled, you don’t have to worry about the white buttercream smearing or getting messed up!
Once you’re happy with your stripes, run a sharp paring knife around the top of the cake to slice off any excess frosting and make the top perfectly smooth. You can heat the knife in hot water to make this easier and neater, if you’d like.
Finally, it’s decoration time! Add your candy balloons to the top, then gently press a ring of sprinkles and candy around the edges of the cake.
Like most cakes, this one is easiest to slice when cold, but has the best taste and texture when enjoyed at room temperature. Leftovers can be kept well-wrapped in the refrigerator for up to a week, but cake is always best when enjoyed within a few days.