Butter Custard Filling

2 1/2 cups milk
1/2 cup granulated sugar
1/2 cup flour
1/4 teaspoon salt
2 teaspoons vanilla extract
1 cup butter

Heat milk in double boiler. Stir hot milk gradually into the sugar, flour and salt mixed together. Return to boiler; cook about 10 minutes, stirring constantly. Cool; add vanilla extract. Cream butter; add the custard to butter gradually, stirring until smooth.