In large bowl beat shortening and butter with electric mixer until light and fluffy. Add vanilla and mix well. Gradually add sugar, one cup at a time, beating well at medium speed. Scrape sides and bottom of bowl often. When all sugar mixture has been mixed in, icing will appear dry.
In small bowl combine milk with cornstarch, stir until cornstarch is dissolve. Add to icing mixture beat at medium speed until lightly and fluffy. Keep icing covered with a damp cloth. Makes 3 cups