In large bowl, beat shortening and butter with electric mixer until light and fluffy. Beat in vanilla.
Gradually add sugar, about 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.
Gradually add milk and beat at medium speed until light and fluffy.
Notes
This recipe is for stiff consistency buttercream, which is excellent for piping decorations like flowers. However, it will need to be thinned for icing cakes and borders. Add milk in small quantities until desired consistency is achieved.