The cake mix in this recipe is a Dark Chocolate Cake Mix from The Pure Pantry that I received for review. The Pure Pantry offers gluten free, organic products such as cake and cookie mixes, pancake mixes, and all purpose baking mixes. Please note this frosting recipe is NOT gluten free.
Cream shortening and butter with electric mixer. Add peanut butter 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, one tablespoon at a time, and beat at medium speed until light and fluffy.
Keep icing 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. Re-whip before using.