1 tsp. dry mustard
2 tsp. sugar
1/4 tsp. salt
2 tbsp. flour
1/2 c. cold water
2 egg yolks
1/4 c. vinegar
2 tbsp. butter

Dissolve mustard, sugar, salt and flour in water. Beat eggs and vinegar in the top of a double boiler; add dissolved ingredients. Cook and stir over boiling water until thick and smooth; add butter.