In a bowl of a standing mixer (or a large bowl with a hand mixer), beat butter until light and fluffy, about 1-2 minutes. Next, add in granulated sugar and beat until fluffy (about 1 minute).
Add in the egg yolks, one at a time, beating well after each addition. Set aside.
In a separate bowl, sift all purpose flour, cornstarch, cocoa powder, baking powder, baking soda, and salt together. Set aside.
Mix buttermilk, vinegar, and vanilla extract together.
Alternately add the dry ingredients and the wet ingredients into the butter mixture, beginning and ending with the dry ingredients, mixing until just combined. (I add 1/3 of the dry ingredients to the butter mixture, then mix until *just* combined. Then I add half of the wet ingredients, and mix until *just* combined. Then I repeat the steps.)
Add in sour cream and food coloring, mixing until just combined.
Lastly, gently fold in the egg whites until the whites are no longer visible.
Fill each muffin cup 3/4 of the way full. (This batter makes about 12-13 cupcakes.)
Bake for 15-17 minutes, or until a cake tester comes out clean.
Allow the cupcakes to cool for a few minutes in the muffin tin, then carefully remove the cupcakes and allow them to continue cooling on a wire rack.