1 egg
2 tbsp. sugar
Pinch of salt
2 tsp. cornstarch
2/3 tsp. dry mustard (make smooth paste with water)
1/2 c. water
1/3 to 1/2 c. vinegar
Lg. lump of butter

Beat egg with sugar, salt and cornstarch. Add dry mustard, water, vinegar and butter. Cook over low heat, stirring constantly until thick. Chill until cold.