Rainbow cake can be made a number of ways, giving you different effects based on the pan you choose. I chose the bundt pan because the shape of the pan, with the flute in the middle, causes the cake to bow, creating a rainbow without having to use several pans or reuse the same pan. Given that bundt pans vary in intricateness, I chose a more traditional pan, with minimal features.
I used a boxed White Cake mix for the batter. You can use a Yellow Cake for the base, but I find it harder to keep the richness of the colors. Since bundt pans are usually quite large, it took two boxes of cake mix, with a little leftover. Prepare your batter as directed, as you will be baking the cake normally, with no special instructions.
Once your batter is prepared, we're going to separate a portion into a bowl and begin adding our food coloring. At this juncture, we must decide which color we want to start with. I began with purple and worked backwards to red, in the "Roy G. Biv" order.
I chose to use water based food dye, since we're not worried about the balance of wet to dry ingredients. The cake batter that goes on the bottom of the pan is going to spread up the sides of the pan, we want this layer of color to be large enough to cover the bottom of your pan by quite a bit.
When you begin to layer your colors on top of one another, the only advice I have is try to keep a steady, even flow of batter that gently pours. Excessive force of the batter with cause the colors to intermix and displace each other, which doesn't make a pretty rainbow, but would make and interesting marbled rainbow. If you mess up, swirl a toothpick around, not too much mixing, to create a completely different effect, but still cool. No reason to waste perfectly good cake.
As you can see, I made my purple layer too thin, and I switched my indigo and blue layers, but still tastes fine.
Now for the Vanilla Vodka Royal Icing
I used a small sauce pan to cook off some of the alcohol in the vodka before added a splash of heavy whipping cream and confectioner's sugar. I didn't really measure my ingredients, but I would say probably 1/4 to 1/2 a cup of vodka and 3 to 4 cups of confectioner's sugar. I would recommend adding the confectioner's sugar slowly, working it in and adding more as needed. The desired texture is entirely up to you, I preferred it to run off the spoon, but still maintain it's shape on the plate.
Serve and Enjoy!