Courtesy of Mckmama's blog and Tasty Kitchen
2 boxes white cake mix (I used butter cake mixes)
Food Coloring (I used the liquid, but next time I'll get the gel colors)
4 packages (8 oz) cream cheese
1 cup butter
5-6 cups powdered sugar
1 tablespoon vanilla
Mix one box of the cake mix according to the package instructions. Separate the batter into three even bowls - I used a 1/2 cup measuring cup to separate evenly and ended up with about 1 1/2 cup of batter per layer.
Mix each bowl with the food coloring for the desired layer - making purple, blue and green first. Bake the cakes in round pans according to the package instructions, but make sure you check them about 10-15 minutes before it is supposed to be finished baking. With less mix in each pan, they cook faster.
Repeat for the remaining colors - yellow, orange, and red. You will have 6 layers of different colors.
(I used store bought frosting for this first time... but next time I will attempt to make this frosting instead)
For the frosting, mix together softened (room temperature) butter and softened (room temperature) cream cheese until well combined. Add vanilla. Combine. Slowly add in powdered sugar.
After the 5th cup of powdered sugar, taste the frosting and see if you like it sweeter. Use 5 if you like it less sweet or 6 if it isn't sweet enough.
To assemble the cake:
Try to level each layer of the cake with a large bread knife. For the rainbow cake especially, if all the layers are level, they they will evenly display when you cut into it. (I didn't do this, and no one gasped that the layers weren't even - if they even noticed.)
Start with the purple layer first. Put the purple layer on your cake stand or plate. Add enough frosting to cover the layer and some to spill over the edges, anywhere from 1/2 to one cup of frosting. Run the flat, straight side of the knife around the cake to smooth out the edges.
Add blue layer of the cake. Put frosting in the middle, start spreading out frosting with the flat, straight side of the knife so that it coversthe cake and falls out over the edges. Then run your knife around the outside edge of the cake to smooth out the sides.
Put the cake into the fridge for at least 2 hours (I only did about an hour) or freeze for 1 hour.
Repeat with the green layer, then the yellow layer.
Put the cake into the fridge for at least 2 hours or freeze for one hour. (Again - only an hour)
Repeat with the orange layer, and then the red.
Cover cake with frosting and run the flat, straight edge of a knife around the edges to smooth out the frosting.
It is important to freeze or refridgerate the cake between layers for a couple of reasons. The cake can get really heavy by the time you add the 4th, 5th, and 6th layers, but if it is somewhat frozen at this point, it will not get smooshed, or crushed. Also, it is much easier to frost the cake when it is solid, or not wobbling.
Once you have finished frosting the cake, add any decorations or words that fit your occasion.
I served this at our church family game night - and it was a BIG hit with the kids!