Preheat oven to 350 degrees.
Grease sides and bottom of each pan with nonstick spray. Flour baking pan lightly. (If making cupckakes, use baking cups in place of the grease and flour)
Blend cake mix, milk, melted butter, egg whites, and vanilla in a large bowl at low speed until moistened for about 30 seconds.
Beat at medium speed for about 2 minutes.
Pour batter in pans and bake according to box directions for your type of baking pan. I tend to check my cakes at the lower end of the time indicated as my oven cooks a bit faster than some others I have had.
Cake will be done when toothpick inserted in center comes out clean. Cool in pan on wire rack for 15 minutes before removing. Cool completely before frosting.