Rainbow cake with pink frosting

Active: 20 minutes; Total: 40 minutes; Yield: 10 to 14 servings

Flour, cake	24 ounces
Sugar, granulated	24 ounces + 8 ¾ ounces
Salt	1 teaspoon + Pinch
Baking powder	2 tablespoons
Baking soda	1 teaspoon
Egg whites	10 ounces + 7 ounces
Oil, vegetable	6 ounces
Buttermilk, room temperature	8 ounces + 10 ounces
Butter, unsalted, room temperature	12 ounces + 16 ounces
Vanilla extract	1 tablespoon + 2 teaspoons
Raspberries	10 ounces
Cream of tartar	¼ teaspoon
Food coloring gel	½ teaspoon per color layer

1. Preheat oven to 335°F; lightly butter and flour six 8-inch-by-2-inch cake pans, and place a round piece of parchment paper in the bottom of each pan. 
2. Combine 8-ounces buttermilk and the oil; set aside. 
3. Combine the remaining buttermilk, 10-ounces egg whites, and vanilla; whisk to integrate the eggs; set aside. 
4. Combine the flour, 24-ounces sugar, baking powder, baking soda, and salt in the bowl of a stand mixer with the paddle attachment; mix 10 seconds. 
5. Add the butter to the dry ingredients and mix on low until the mixture resembles coarse sand, about 30 seconds. 
6. Add milk-oil mixture, and mix until the dry ingredients are moistened, then increase speed to medium (setting 4 on a KitchenAid), and mix for 2 minutes. 
7. Scrape your bowl, and reduce speed to low. 
8. Add the egg-milk mixture in three batches, letting the batter mix for 15 seconds between additions. 
9. Scrape down the sides again to ensure everything has incorporated. 
10. Divide the batter into six bowls; weigh 15 ounces of batter for each bowl. 
11. Add ½-teaspoon food coloring (red, orange, yellow, green, blue, purple) to each bowl. 
12. Bake for 20 to 24 minutes, or until the edges just start to pulling away from the cake pan. 
13. Immediately tap the pan firmly on the countertop once to release the steam of the cake; this stops the cake from shrinking. 
14. Cool for 10 minutes inside the pans before flipping out. 
15. Freeze for 30 to 60 minutes before frosting. 
16. Press the raspberries through a sieve, discard the seeds; set purée aside.  
17. Using a hand whisk and the stand mixer bowl, whisk together 7-ounces egg whites, 8 ¾-ounces sugar, and cream of tartar.  
18. Place the bowl over (not in) a saucepan of simmering water.  
19. Gently whisk the mixture until it is very hot (130°F on a candy thermometer).  
20. Move the bowl to the stand mixer and, using the whisk attachment, whip the whites on medium-high speed until they have tripled in volume and are thick and glossy and hold stiff peaks, 3 to 4 minutes.  
21. Reduce speed to medium-low, and continue for another 1 to 2 minutes, until the bowl is just cool to touch.  Increase speed to medium-high, and add 16-ounces butter, one piece at a time, adding the next piece just as the previous one is incorporated.  
22. Stop the mixer every so often to scrape down the sides.
 At some point, the buttercream appears curdled; don’t worry, this is normal.  
23. After all the butter is incorporated, and the frosting is light and fluffy, add the purée, 2-teaspoons vanilla, and pinch of salt.  
24. Rewhip the frosting by hand or with a mixer just before frosting the cake. 
25. For the crumb coat, top the purple layer with ¾-cup frosting, and spread to the edges for about ¼-inch thickness; repeat with the blue, green, yellow, orange, and red layers, then apply a thin layer of frosting all over; refrigerate 15 minutes. 
26. Frost with the remaining frosting. 

Modified recipe by Liz Marek on Sugar Geek Show.