red, orange, yellow, green, blue, and purple gel food color
For the Frosting:
2cupsunsalted butter(at room temp)
4 ½cupsicing sugar
milk(use to thin the frosting)
In the bowl of a stand mixer blend together the dry ingredients.
In a separate container mix the wet ingredients until combined and add them to the dry ingredients. Mix until just combined being careful not to over mix.
Evenly distribute batter between 6 bowls and color one each red, orange, yellow, green, blue, and purple using about a pea sized amount of color.
Fill lined muffin tins ⅔ full and bake at 350 degrees for 20 minutes or until a toothpick inserted into a center cupcake comes out clean.
Allow to cool completely before slicing and assembling. Once cooled (for best results freeze for 10-15 minutes first) remove the rounded top of the cupcake and set aside.
Then cut 3 even slices about ¼" thick from each cupcake. Be careful to keep them stacked in order of top, middle, and bottom since a cupcake is slightly wider at the top than the bottom. This will be helpful for assembly.
To assemble start with a purple layer, add about 1 Tbs of frosting, then the blue layer, and repeat in the reverse order of the rainbow ending with red. It's helpful to make your cakes line up and avoid falling over if you match up all the bottom pieces, all the middle pieces, and all the top pieces.