Cream margarine, add some icing sugar and continue whipping. Add vanilla and some cream. Whip. Add remaining cream if icing seems too thick. Whip until light and fluffy. VARIATION Chocolate butter icing. Melt 1 square unsweetened chocolate with margarine, or sift 1/2 cup cocoa with icing sugar and increase margarine to 5 tablespoons. Follow the above method. YIELD: 1 1/4 cups