emma's rainbow birthday cake!

Recipe courtesy of Martha Stewart

Ingredients

3 cups self raising flour
1/2 teaspoon salt
226g softened unsalted butter
2 1/3 cups sugar
5 large egg whites
2 teaspoons pure vanilla extract
1 1/2 cups milk
Red, orange, yellow, green, blue, and purple food coloring

Buttercream or meringue icing

1.Preheat oven to 180 degrees. Grease six 9 inch round cake pans. Line bottoms with parchment paper; brush again and set aside.

2.In a large bowl, whisk together flour, baking powder and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar. Slowly add egg whites and mix until well combined. Add vanilla and mix until fully incorporated. Add flour mixture and milk in two alternating additions, beginning with the flour and ending with the milk. Mix until well combined.

3.Divide batter evenly between six bowls. Add enough of each color of food coloring to each bowl, whisking, until desired shade is reached. Transfer each color to an individual cake pan. Transfer to oven and bake about 15-20 minutes.

4.Remove cakes from oven and transfer to a wire rack; let cool for 10 minutes. Invert cakes onto a wire rack; re-invert and let cool completely.

5.Using a serrated knife, trim tops of cakes to make level. Place four strips of parchment paper around perimeter of a serving plate or lazy Susan. Place the purple layer on the cake plate. Spread a scant 1 cup buttercream filling over the first layer with a small offset spatula so it extends just beyond edges. Repeat process with blue, green, yellow, and orange layers.

6.Place the remaining red layer on top, bottom-side up. Gently sweep away any loose crumbs with a pastry brush. Using an offset spatula, cover the top and sides with a thin layer of icing. Refrigerate until set, about 30 minutes.

7.Using an offset spatula, cover cake again with remaining frosting.