Combine 3 cups powdered sugar, butter, shortening, 4 tablespoons milk and vanilla in large bowl. Beat with electric mixer until smooth. Add remaining powdered sugar; beat until light and fluffy, adding more milk, 1 tablespoon at a time, as needed for good spreading consistency. This makes about 3 1/2 cups.
Notes
Great for decorating cakes, and it tastes good too! (We did find that "roses" tend to wilt at room temperature because of the butter.)