The first variation of a rainbow cake that I ever encountered was one in which each cake layer was a different color of the rainbow. Yes, that would be 6 layers of cake with plenty of sugary frosting between each layer and all over the sides and top of the cake. I knew this size would be over the top for our family of three, even if we did share with our friends and neighbors, so I began a search for a single layer version. One day I do plan to make the ultimate 6 layer rainbow cake (or at least a version of it) but for now, this single layer cake with do just fine for us.
After mixing the golden vanilla cake mix according the directions on the package, I separated the batter into six bowls. Being the type A baker that I am I felt the need to somewhat measure my portions so that I could make sure that I had successively smaller amounts of batter for each color. You can see from the picture that my largest color was red, followed by orange, yellow, green, blue, and finally violet. There is about a 2:1 ratio of red to violet. Once I separated the batter I added the appropriate food color. Gels work the best to get the most vivid colors.
The next step was to pour the batter into the springform pan. I poured the red batter into the center of the pan and then tapped it on the counter to get it spread out to about an inch or two from the edge. Each color you add will push the previous layers out so you don’t need to go all the way to the edge with the red. Pour each successive color and tap between layers to spread out the color. Tap just a few times for green, and no taps for blue or violet. I would suggest that you put a cookie sheet below your springform pan when you bake it since the cake expands out the crack in the bottom of the pan.
Cool the cake completely before frosting. Did you know that you can cool your cake in the freezer if you don’t want to wait all day for it to cool on the countertop? When it does come time to frost, choose a neutral or plain colored frosting. The beauty of this cake is on the inside so it’s fun to use a plain color or even gray frosting so that you can take your cake eaters by surprise! I used both chocolate and vanilla and did yin and yang since I love chocolate and Ron loves vanilla; the best of both worlds. Enjoy!
Updated to add that you can also do cupcakes and mini cupcakes. A LOT more work than a single layer cake though, so be prepared.
Rainbow Cake (Single Layer)
1 box golden vanilla cake mix (you can also use vanilla, yellow, or white)
1 cup water (follow the amount of water on the back of the box)
1/2 cup vegetable oil (follow the amount of water on the back of the box)
gel food coloring
Preheat oven to 350 degrees F. Grease and flour a 9-inch springform pan.
Prepare cake mix according to directions on box. Divide batter into six bowls. Measure out the batter so that the red has twice as much as the batter that will be violet. (For example, if there is 1 1/2 cups of batter for red then you will have about 3/4 cup of batter for violet. Once all the batter is divided out, add the appropriate food coloring (gel coloring works best for vibrant colors).
Pour the red batter into the bottom of the springform pan. Tap the pan on the counter a few times (if necessary) to spread the batter to within an inch or two of the edge. Pour the orange batter directly onto the center of the red batter. Tap the pan; the red batter will continue to spread as the orange batter spreads. Continue with all colors: yellow, green, blue, then violet. For green you shouldn’t need to tap much, and for blue and violet you shouldn’t need to tap at all.
Place cake on a cookie sheet to protect your oven from dripping. Bake according to directions on the box. (Check for “doneness” often since it is a double layer cake being baked as a single layer. I checked mine at 25 minutes, and then every 5 minutes or so after that for a total of about 38 minutes).
Cool for about 10 minutes in pan. Remove from pan and cool completely on a wire rack. Frost when completely cooled.
Makes one 9-inch cake