Preheat oven to 350°F. Line muffin pans with cupcake liners. Whisk the flour, cornstarch, baking powder, baking soda, and salt together in a large bowl.
1 1/2 cups all-purpose flour, 1/4 cup cornstarch, 1 teaspoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon salt
Beat the butter and sugar on high speed in a stand mixer or using a handheld mixer for 3 minutes until creamy and light. Scrape sides and bottom of the bowl.
1/2 cup salted butter, 1 cup granulated sugar
Add egg whites and vanilla, beating on medium-high speed until combined. Beat in sour cream.
3 egg whites, 1 Tablespoon pure vanilla extract, 1/2 cup sour cream, 1/2 teaspoon almond extract
Add the dry ingredients, mixing on low speed just until combined, then scrape the bottom and sides of the bowl.
With the mixer running on low speed, add the milk, stirring just until combined. Do not overmix.
1/2 cup whole milk
Fill each cupcake liner about 2/3 full with batter, then bake for 19-22 minutes, until a toothpick inserted in the center of a cupcake comes out clean.
Cool completely before frosting.