Cook 3 level tablespoons flour with 3/4 cup water until thick. Stir, so it won't lump. Beat together 3/4 cup (1 1/2 sticks) butter, 3/4 cup sugar, 1/4 teaspoon salt and 1 1/2 teaspoon vanilla. When smooth, add to paste. Beat well. Chill before spreading on cake.