This rainbow layer cake is pure joy! Vibrant inside and out, just slice into this multi-colored cake and eat the rainbow!
Preheat the oven to 350 degrees. Grease two 8-inch round cake pans and cut out six parchment rounds to line the bottom of the pans. Set aside.
Sift together the flour, baking powder, baking soda and salt. Set aside.
Beat the butter with an electric mixture on high. Gradually add in sugar and beat until fluffy, about 2 mins.
Add the egg whites and beat until well blended, about 2 mins, then add in the sour cream and beat until combined.
Add in one third of the flour mixture and mix until just combined. Add one half of the milk, mix until combined. Continue with the flour, milk, and finish with the flour. Don't over-mix. Stir in the vanilla.
Divide the batter evenly into six bowls (about 1 1/3 cups each). Add about 3-4 drops of the red, orange, yellow, green, blue, and purple gel food coloring into each bowl to create a different color layer cake.
Pour one color batter into each of the prepared pans and bake for about 12 minutes or until a toothpick inserted into the center comes out clean. (You will need to repeat twice for the other layers or use additional cake pans if you have them.)
Cool cake in pans for a couple minutes, then remove from pan, peel off the parchment rounds and finish cooling on a wire rack. Repeat with remaining colored batter.
Whip butter on high speed with a whisk attachment until fluffy, 2-3 mins.
Gradually add confectioners sugar and mix until combined. Add vanilla and whip for 3-5 mins on high speed until light and fluffy.
Reserve 1/2 of the white icing and separate the remaining buttercream into 6 small bowls. Use a drop or two of gel food coloring in each to reach desired color. Stir well to combine.
Place the purple cake layer on a cake plate and spread 1/6 of the white frosting over the cake layer. Repeat with remaining cake layers in this order: purple, blue, green, yellow, orange, red (red will be the top layer).
Spread a very thin layer of white buttercream over the cake, just enough to give the piped decorations something to adhere to (it doesn't need to completely cover the cake.)
Fit six disposable pastry bags with a round tip (#12) or simply cut a small hole and fill each with a different color buttercream. Pipe one different color dot in a vertical row in this order from top to bottom: red, orange, yellow, green, blue, purple.
Use a small offset spatula and smear half of each dot straight across. Repeat, alternating colors in order (purple next to red, red next to orange, orange next to yellow, etc.). Smear the dots and continue until the sides are iced.
Follow the same technique for the top of the cake, creating concentric circles.