Melt butter in a small saucepan set over medium heat. In a medium-size bowl stir flour with sugar. Then whisk into melted butter until mixture is moistened. Stir in milk until smooth. Cook whisking frequently until thickened and smooth; about 5 minutes. Stir in brandy and vanilla. Remove from heat.
Serve right away over a cake or cover and refrigerate for a week or more. Reheat just before serving.