1 tablespoonful flour. 1 tablespoonful butter. 1 tablespoonful mustard. 1/2 tablespoonful sugar. 1 teaspoonful salt. 1 egg. 3/4 cup milk. 3/4 cup vinegar. Use a double boiler, put in it the first five articles, stir together until smooth; add the well-beaten egg and the milk. Let cook, stirring hard. Then add vinegar, and beat all with an egg-beater until the mixture is smooth and creamy. Let cool before using.