In a large mixing bowl, cream butter and shortening with an electric mixer. Add vanilla.

When all the sugar has been mixed in, icing will appear dry. Add milk and beat at high speed until light and fluffy.

Gradually add sugar one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often.

Keep covered with a damp cloth until ready to decorate. For best results, keep icing cool in refrigerator when not using.