1 and 3/4 cups (200g) sifted cake flour
3/4 teaspoon baking powder
1/4 teaspoon baking soda
1/4 teaspoon salt
1/2 cup (115 g) unsalted butter, softened to room temperature
1 cup (200 g) granulated sugar
3 large egg whites, at room temperature
1/2 cup (120 g) full-fat sour cream, at room temperature
2 teaspoons pure vanilla extract
1/2 cup (120 ml) whole milk, at room temperature
For vanilla buttercream :
1 and 1/2 cups (3 sticks; 345g) unsalted butter, softened
6 cups (580 g) confectioners sugar
1/3 cup (80 ml) heavy cream or whole milk
2 teaspoons pure vanilla extract
1/8 teaspoon salt
Green, blue, yellow and purple food color (optional)