The best way to make cake icing from powdered sugar is to use 3 cups powdered sugar, 1/3 cup softened, unsalted butter, 1 1/2 teaspoons pure vanilla extract and 1 to 2 tablespoons whole milk. Combine these ingredients thoroughly to make the icing.
Cream the butter and the powdered sugar in a medium bowl with either a wooden spoon or an electric mixer, either standing or hand-held. Add the vanilla and 1 tablespoon of the milk and blend until well-incorporated. Continue to add more milk a few drops at a time until the frosting can be easily worked. If the frosting becomes too thin, add enough powdered sugar to make it spreadable. If it becomes too sweet, add a couple of drops of milk.
This recipe takes about 10 minutes and frosts one 13 by 9 inch cake or an 8- or 9-inch two layer cake. One recipe is enough to fill in the space between the layers.
Another version of this frosting is to add 2 tablespoons of cocoa powder to the mix for chocolate frosting. Beat the mix until fluffy. Make sure the cake is completely cooled before frosting. Use an offset spatula when applying to the cake for a very smooth frosting.