* Note: If tinting frosting, use powder form of food coloring, available where cake decorating supplies are sold. For pure white icing, omit the butter and use 1 cup shortening and 1/2 teaspoon butter flavoring (use clear flavoring; some are tinted yellow).
Beat together butter and shortening with electric mixer. Add vanilla. Gradually add sugar, 1 cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often.
When all the sugar has been mixed in, icing will appear dry. Add milk; beat at medium speed until light and fluffy.
Keep icing covered with a damp cloth and refrigerated until ready to use. Refrigerated in an airtight container, icing may be stored for 2 weeks. Rewhip before using.
Yields 3 cups; enough for about 15 or 20 cake squares.