1/4 c sugar
1/4 c plain flour
2 tsp salt
2 tsp dry mustard
1 1/2 c milk
1/2 c mild vinegar or lemon juice
4 egg yolks, slightly beaten
1 tbsp butter

Mix sugar, flour, salt and mustard in saucepan. Gradually blend in milk and vinegar. Cook over direct low heat, stirring constantly until mixture thickens and boils. Boil 1 minute. Stir at least half of hot mixture into egg yolks. Stir into remaining hot mixture in saucepan. Bring to boil, stirring constantly. Boil 1 minute. Remove from heat and stir in butter. Cool. 2 cups dressing. When using mix with an equal amount of whipped cream, sour cream or mayonnaise.