1/2 c. sugar
1/4 c. + 1 tbsp. dry mustard
2 tbsp. all-purpose flour
1 c. milk
1 egg yolk
1/2 c. vinegar

Combine sugar, mustard, salt, and flour in a small saucepan; stir well. Combine remaining ingredients, beat well. Gradually add milk mixture to mustard mixture; cook over low heat, stirring constantly, until thickened and bubbly. Cool thoroughly, and store in refrigerator.