Hester's Red Velvet Cake Icing

Cook the milk and flour in the top of a double boiler over just-barely simmering water, stirring frequently and scraping down the sides with a rubber spatula until thickened, about 5 minutes. Place the mixture in the refrigerator until chilled. Combine the vegetable shortening, sugar, and vanilla extract with an electric mixer until well blended. Beat the chilled milk mixture into the shortening mixture until fluffy.