Confetti Cake by Smitten Kitchen

https://smittenkitchen.com/2017/07/confetti-party-cake/

CAKE - 1 layer 9 inch round (see below for doubled)

• 1/2 cup unsalted butter, at room temperature
• 2/3 cups granulated sugar
• 1/2 teaspoon kosher salt
• 2 large egg whites
• 1/2 teaspoon vanilla extract
• 1/2 cup buttermilk
• 1 teaspoon baking powder
• 2 tablespoons (16 grams) cornstarch
• 1 cup (130 grams) all-purpose flour
• 1/4 cup rainbow sprinkles

FROSTING - for 1 layer party cake

• 1/2 cup (115 grams or 4 ounces) unsalted butter, at room temperature
• 1 1/4 cups (150 grams) powdered sugar, sifted if lumpy
• Pinch of fine salt
• 1/2 teaspoon vanilla extract, plus more to taste
• 1 tablespoon milk or cream
• Rainbow sprinkles for decoration

Preheat oven: to 350 degrees F. Line the bottom of an 8×8-inch square or 9-inch round cake pan with parchment paper and coat the bottom and sides with nonstick spray or butter.

Make cake: 

1. Beat butter, sugar, and salt together in a medium bowl. 
2. Add egg whites, one at a time, beating until combined and slightly fluffy. 
3. Add vanilla and buttermilk and beat to combine (the mixture will instantly look like cottage cheese and you will be sure it’s ruined but I promise it is not). 
4. Add baking powder and cornstarch and beat very well to combine. 
5. Scrape down sides of bowl and beat one second more. 
6. Add flour and mix just until it disappears. 
7. Use a rubber scraper to gently fold in sprinkles.

Bake cake: Spread batter in prepared pan and smooth the top. Bake for 20 minutes, or until a toothpick inserted into the center of the cake comes out batter-free. Let cool in pan on cooling rack for 5 minutes, then run a knife around the side to loosen the cake, flip it out onto the cooling rack, and slide the cooling rack into the freezer until cake is cool, about 10 to 15 minutes.

Frost and serve: While the cake cools, beat butter, sugar, salt and vanilla until fluffy, then add milk or cream and beat until smooth. Once cake is fully cool, transfer it to a serving plate. Spread frosting on top — you’ll have more than enough, so if you need to set some aside for small decorations or tinting and writing on the cake, this won’t be a problem. Finish with sprinkles. Share with friends.

CAKE 2 layer 9 inch rounds:

1 cup unsalted butter, at room temperature
1 1/3 cups granulated sugar
1 teaspoon kosher salt
4 large egg whites
1 teaspoon vanilla extract
1 cup buttermilk (or 1 cup regular milk with 1 tbsp vinegar)
2 teaspoon baking powder
4 tablespoons cornstarch
2 cup all-purpose flour
1/2 cup rainbow sprinkles

FROSTING - tripled
1.5 cup unsalted butter, at room temperature
3.75 cups powdered sugar, sifted if lumpy
3 pinches of fine salt
1.5 teaspoon vanilla extract, plus more to taste
3 tablespoons milk or cream
Rainbow sprinkles for decoration