Cream shortening and butter with electric mixer. Add cocoa and 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 at medium speed until light and fluffy. Keep bowl covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Rewhip before using.
Chocolate Mocha Icing: Substitute cold brewed strong coffee for milk in Chocolate Buttercream recipe.
Darker Chocolate Icing: Add an additional 1/4 cup cocoa (or one additional 1 oz. square unsweetened chocolate, melted) and 1 additional tablespoon milk to Chocolate Buttercream Icing.
For Thin (Spreading) Consistency Icing: Add 3-4 tablespoons light corn syrup, water or milk.
YIELD: Makes about 3 cups.