Prepare pudding mix according to package directions using the 1 1/2 cups milk.
In a small mixer bowl beat margarine and shortening with an electric mixer on medium speed for 30 seconds. Add powdered sugar to beaten mixture; beat till lightly and fluffy. Gradually add pudding, beating well.
Makes 3 3/4 cups, enough to frost top and sides of one 13x9x2-inch cake or tops and sides of two 8-or9-inch cake layers.
Source: Better Homes and Gardens Creative Ideas: Holiday Desserts, 1988.