Beat butter and shortening until light and fluffy. (The butter should be at room temperature. If they are too soft, return to the refrigerator for a few minutes.)
Add the vanilla.
Gradually add the confectioner's sugar. Continue beating until the frosting is smooth and creamy.
You may not need all the confectioner's sugar. If your frosting becomes to thick, gradually add a teaspoon of milk at a time until you reach the desired consistency.