This recipe will generously frost a two-layer 8-inch or 9-inch cake. For an 8-9-inch sheet cake, I would cut the recipe by one quarter. This is a light buttercream, so for multiple-layer cakes, I would increase the sugar by up to a cup to give more structure. For frosting cupcakes: The recipe yields enough buttercream to frost 18 cupcakes generously, or lightly frost 24 cupcakes.