Melt butter in a small saucepan over moderately low heat, blend in flour, add milk slowly, cook and stir until thickened and smooth. Mix in salt, sugar, and mustard paste. Add vinegar, 1 tablespoon at a time beating well after each addition. Cool dressing, then cover and chill 2 to 3 hours. Beat well before using. Use to dress any cooked vegetable or seafood salad. Dressing keeps well (about a week) in the refrigerator.