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, icing will appear dry. Add milk and beat at high speed until light and fluffy. Keep covered with damp cloth until ready for use. Refrigerate when not in use. Keeps for up to 2 weeks in airtight container. Rewhip before using.