Corrina's Butter Cream Icing

Cream butter and shortening with electric mixer. Add vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add milk and beat a medium speed until light and fluffy. Keep icing covered with a damp cloth until ready to use. For the best results, keep icing bowl in refrigerator when not in use. Refrigerate in an airtight container, this can be stored up to two weeks. Rewhip before using.