In a large bowl, cream together the confectioners' sugar and shortening until smooth.
Gradually mix in the milk, vanilla, and butter flavor extract with an electric mixer until smooth and stiff, about 5 minutes. 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 icing bowl in refrigerator when not in use. Refrigerate in an airtight container, this icing can be stored. Re-whip before using.
For Thin Consistency: add 2 tablespoons corn syrup, water or milk.
Pure White Icing (Thick Consistency) Omit butter; substitute an additional 1/2 cup shortening for butter and add 1/2 teaspoon no-color butter flavor. Add up to 4 tablespoons light corn syrup, water or milk to thin icing for cakes.
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