Cream butter and shortening with electric mixer. Add vanilla. Gradually add sugar, one cup at a time, beating on medium speed. Scraping sides of the bottom of the bowl often. When all sugar has been mixed, icing will appear dry. Add milk and beat at medium speed until light and fluffy. For best results, keep icing covered with a damp cloth bowl in refrigerator when not in use. Refrigerate in an airtight container, icing stores for 2 weeks. Makes 3 cups.