From America's Test Kitchen
Makes 1 1/2 cups, enough for 12 cupcakes
Any of the buttercream frostings (see related recipes) can be made ahead and refrigerated; if refrigerated, however, they must stand at room temperature to soften before use. If using a hand-held mixer, increase mixing times significantly (at least 50 percent). This recipe can be doubled to make enough for a two-layer cake.