This is for baking over Christmas holidays when you just don't want to run out of icing.
With an electric mixer on low speed, beat all ingredients until smooth, about 7 minutes. If icing is too thick, add more water, a little at a time, beating until icing holds a ribbon on the surface for a few seconds when beater is lifted; if too thin, continue mixing 2 to 3 minutes more. Use immediately, or refrigerate in an airtight container up to 1 week; stir well with a flexible spatula before using.