In a large mixing bowl cream butter and shortening with electric mixer. Add vanilla.
Gradually add in sifted confectioners sugar, one cup at a time beating on medium speed.
Scrape sides and bottom of bowl often. When all of the sugar has been mixed in, icing will appear dry. Add milk and beat at medium speed for 3 minutes.
Keep icing covered with a damp cloth until ready to decorate. For best results, keep icing bowl in refrigerator when not using.
Icing can be stored in an air tight container for up to a week. Just re-whip it in your mixer before using. Makes 3 1/2 cups.
Notes
Store at room temperature for up to 3 days.
Store in the refrigerator in an airtight container for up to 1 month.
*Nutritional information is a calculated guesstimate. Please note that this can change with different brands and modifications you may make to the recipe. For the most accurate information, use a nutritional calculator with the exact brands and measurements you're using with each recipe.