In a small saucepan, cook flour and milk until thick, stir well. Set aside to cool, (keep mixture covered as it cools). In a bowl beat sugar, margarine (or butter) and shortening using high speed of mixer, until fluffy. Add the milk mixture and vanilla. Continue beating until the icing is very fluffy.
Ingredients
1 cup milk
1/4 cup flour
1 teaspoon vanilla
1 cup sugar
1/2 cup butter or margarine
1/2 cup shortening