Preheat the oven to 350 degrees Line a 12-cup standard size cupcake tin with paper liners. Spritz each with nonstick cooking spray.

Lightly whisk milk, egg whites, and vanilla extract in large glass measuring cup.

Mix cake flour, sugar, baking powder, and salt in the bowl of a stand electric mixer and add butter; beating at low speed until mixture resembles moist crumbs, with no large clumps of flour.

Add 3/4 of the milk mixture to the crumbs and beat at medium speed (or high speed if using handheld mixefor 1 1/2 minutes. Add remaining milk mixture and beat 30 seconds more. Scrape down the sides of the bowl and beat for 20 seconds longer at low speed.

Evenly distribute the batter in the cupcake tin. Bake for 20-22 minutes, or until a toothpick inserted in the center comes out clean.

Remove the cupcakes from the pan carefully and let them cool completely on a wire rack.

Preheat the oven to 350 degrees Line a 12 cup standard sized cupcake tin with paper liners and spritz lightly with nonstick cooking spray.

Whisk flour, sugar, baking powder, and salt in the bowl of a standing electric mixer. Add butter, sour cream, eggs, and vanilla. Beat at medium speed until smooth and satiny, about 30 seconds. Scrape down sides of bowl with rubber spatula and mix by hand until smooth and no clumps of flour are visible.

Evenly divide the batter among the cups in your prepared pan. Bake until cupcake tops are pale gold and a toothpick inserted into the center comes out clean, 20 to 22 minutes. Remove the cupcakes from the pan carefully and let them cool completely on a wire rack.