|Photos Courtesy of ~ Mrs. Regueiro's Plate|
Rainbow Cake with Lemony Swiss Buttercream
Adapted from Martha Stewart
3 cups all-purpose flour
4 teaspoons baking powder
½ teaspoon salt
1 cup unsalted butter, (2 sticks) room temperature
2 ⅓ cups sugar
5 large egg whites, room temperature
2 teaspoons vanilla extract
1 ½ cups milk, room temperature
Food Gel Coloring: Red, orange, yellow, green, blue, purple (I used pink...but I added too much)
Lemony Swiss Meringue Buttercream
10 large egg whites
2 cups sugar
2 cups unsalted butter, (4 sticks) room temperature
2 teaspoons lemon extract
Preheat oven to 350 degrees. Coat the six 8-inch round cakes pans with shortening (or whatever amount of cake pans that you have. I only have three, reuse what you have). Line each of the cake pans with a fitted parchment paper, coat again with shortening.
In a large bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of the stand mixer, cream together the butter and sugar. Slowly add the egg whites and vanilla extract and mix until fully combined. Alternate adding in the dry ingredients and milk ingredients with the butter mixture. Mix until fully combined.
Make sure you have at least 6 medium size bowls and divide the batter evenly. A food scale would help and I figured about Add enough food coloring to each bowl until the desired shade is reached. Transfer batter to pan. Bake for 15 minutes, or until a cake taster comes out clean.
Remove cakes from oven and transfer to a cooling rack. Let cool for 10 minutes. Invert cakes on cooling rack and cool completely.
To make the frosting, cook egg whites and sugar in a small saucepan over medium heat in a heatproof bowl, whisking constantly until sugar has completely dissolved. Once the temperature has reached 140 degrees, transfer to the bowl of the stand mixer fitted with the whisk attachment. Mix egg white mixture on high speed until it reaches room temperature. With the mixer on low, slowly add butter, one tablespoon at a time. Mix until fully combined. Remove whisk attachment, and switch back to paddle attachment. Increase the speed to medium-high and beat until buttercream comes together, add the lemon extract.
Using a serrated knife, even out the cake and trim off any excess. Place a large piece of wax paper on the serving cake plate or on top of a lazy susan. Place the purple layer on the cake plate. Spread a dollop (½ cup or more) of buttercream filling and spread it evenly with a cake spatula. Repeat process with blue, green, yellow, and orange layers.
Place the remaining red layer on top, bottom-side up. (Which I neglected to do but you can barely tell). Brush away any excess crumbs with a pastry brush or your hands. Using an offset spatula, cover the top and sides with a thin layer of frosting. Refrigerate until set, abut 30 minutes.
Using the offset spatula, cover cake again with remaining frosting. Garnish with colored sprinkles if desired.
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Many thanks to MrsRegueiro.com!