Don't substitute margarine for the light butter in this recipe. The texture and taste will not be the same.
Cook milk and flour in a small heavy saucepan over medium heat 2 1/2 minutes or until very thick, stirring constantly with a whisk. Spoon into a bowl; cover and chill thoroughly.
Beat butter and chilled flour mixture at medium speed of a mixer until smooth. Gradually add sugar, and beat just until blended (do not overbeat). Stir in vanilla; cover and chill.