1 1 In medium bowl, mix powdered sugar and butter with spoon or electric mixer on low speed. 2  Stir in vanilla and 1 tablespoon of the milk. 3  2 Gradually beat in just enough remaining milk to make frosting smooth and spreadable.4  If frosting is too thick, beat in more milk, a few drops at a time. Stir in vanilla and 1 tablespoon of the milk. 2  2 Gradually beat in just enough remaining milk to make frosting smooth and spreadable. 3  If frosting is too thick, beat in more milk, a few drops at a time. 4  If frosting becomes too thin, beat in a small amount of powdered sugar.