Step 1

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.

Step 2

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.

Step 3

Add egg whites and vanilla, beating on medium-high speed until combined. Beat in sour cream.

Step 4

Add the dry ingredients, mixing on low speed just until combined, then scrape the bottom and sides of the bowl.

Step 5

With the mixer running on low speed, add the milk, stirring just until combined. Do not overmix.

Step 6

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.

Step 7

Cool completely before frosting.