In top of double boiler, stir together sugar, flour, mustard and salt. Blend in egg and beat well. Stir in butter, vinegar and milk until smooth. Place over simmering water and cook, stirring constantly, until thick, 10 to 15 minutes. Let cool. Dressing will continue to thicken as it cools. Stir as it cools and store in clean glass jar in refrigerator for up to 1 month. Makes about 2 cups. Typed in Mm Format by chartlin@hotmail. Com