First, take two white cake mixes (quelle horreur!). Prepare them according to the package instructions, and divide the batter among six bowls. I weighed the batter to make sure they were even. Follow the Roy G. Biv rules of rainbows and tint the batter the following colors: red, orange, yellow, green, blue, and purple. For most vivid colors, use gel food colors. You can get some neon colors at the grocery store made by Betty Crocker that would work nicely for this cake. I used my Wilton colors.
Next, prepare two nine-inch cake pans by spraying with Pam, lining with a circle of parchment, and spraying the parchment with Pam. Bake two colors at a time, for 15 minutes. You can bake more at once if you have two ovens or more than two nine-inch pans.
Let the cakes cool and remove parchment.
Now, make the buttercream frosting in this post, but triple it. Layer and frost the cake in the Roy G. Biv pattern. Use a crumb coat to the outside of the cake. If your frosting is too thick for a thin crumb coat, add a small amount of milk to about 1/3 of the remaining frosting. Put the cake in the refrigerator until it crusts over - about 30-60 minutes.
Finally, put the remaining frosting in six bowls. Color the frosting with the six colors you used before. Put frosting into six decorating bags. Using the 103 or 104 petal tip, pipe frosting in a ruffle form. The thin end of the tip should be up. Squeeze a short line of frosting, then go back and forth to create a ruffled effect - think zig-zag. This is actually very easy and forgiving.
Here's a (slightly messy) picture of the cake once you cut into it.
And here's a slice. We had to cut them very thinly because the cake was really tall, with six layers!
1. If you don't want to use the cake decorating tools, you can just frost the outside with white frosting. Here's a rainbow cake done that way. Think of the surprise once you cut into a pure white cake and find a rainbow!
2. I never use cake mixes, but for this cake I wanted the colors to be undiluted, and I was afraid a homemade cake wouldn't be white enough.
3. I actually used a swiss meringue buttercream for the outside. It was fun to make and less sweet than the powdered sugar buttercream I usually use. I'd like work with it a bit more, but for now I'll stick with my usual recipe.
4. The cake shifted a bit in transit, as you can see below.
Happy birthday, Kelsey!