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.