In medium bowl, mix powdered sugar and butter with spoon electric mixer on low speed. Stir in vanilla and 1 tablespoon of the milk.
Gradually beat in just enough remaining milk to make frosting smooth and spreadable. If frosting is too thick, beat in more milk, a few drops at a time. If frosting becomes too thin, beat in a small amount of powdered sugar. Frosts 13 X 9-inch cake generously, or fills and frosts an 8- or 9-inch two-layer cake.