In a large bowl, cream shortening and butter with electric mixer. Add vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrap 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 for 2 weeks. Rewhip before using.
For thin (spreading) consistency icing,
add 2 tablespoons light corn syrup, water, or milk.
For Pure White Icing (stiff consistency),
omit butter, substitute an additional 1/2 cup of shortening for butter and add 1/2 teaspoon no-color butter flavor. Add up to 4 tablespoons of light corn syrup, water or milk to thin for icing cakes.