In a mixing bowl combine the butter and sugar, beat using an electric mixer, 1-2 minutes until fluffy. Add in the egg whites, sour cream, milk, and vanilla extract. Beat lightly with the electric mixer. In a separate mixing bowl combine the flour, baking powder, and salt. Sift the dry ingredients together, this will help produce extra fluffy cupcakes. Gradually add the flour mixture to the wet ingredients a few spoonfuls at a time. Mix until just incorporated. Fill the muffin liners 1/2 full.