-
1
Preheat oven to 350 degrees F. Line six 6-inch cake pans with parchment paper.
-
2
Beat egg whites with an electric mixer on medium-high speed until foamy, about 2 minutes.
-
3
Increase mixer speed to high and continue beating until soft peaks form, about 3 minutes.
-
4
Gradually add 1/2 cup of the sugar, continuing to beat until stiff peaks form, about 3 minutes more.
-
5
In another large mixing bowl, cream butter and the remaining 1/2 cup of sugar on medium speed until light and fluffy and increased in volume, about 5 minutes.
-
6
Add applesauce, vanilla, and eggs and mix on medium speed for 1 minute more.
-
7
Sift together flour, baking powder, and salt.
-
8
Add to butter mixture and mix on medium speed for 1 minute.
-
9
Pour in 3/4 cup water and mix to combine.
-
10
Fold in egg whites with a large spatula.
-
11
Divide batter evenly among 6 small bowls.
-
12
Starting with the lightest color, tint batter with food coloring, whisking in a few drops at a time until the desired intensity is reached.
-
13
Pour each of the colored batters into a different parchment-lined cake pan (each should be half full) and bake until a toothpick inserted in the center comes out clean, about 12 to 15 minutes.
-
14
Stack layers according to the colors of the rainbow, spreading a thin layer buttercream between each.
-
15
Using an offset spatula, frost the whole cake with a thin layer of buttercream.
-
16
Place cake on a freezer-safe plate.
-
17
Freeze for 15 minutes, or until buttercream is firm.
-
18
Remove from freezer and cover cake in sprinkles.
-
19
See how to cover a cake in sprinkles.
