1/2 c. sugar
2 egg yolks
2 tbsp. (level) flour
1/4 tsp. mustard
1/8 tsp. salt
1/4 c. vinegar mixed with 1/4 c. water
1 tsp. butter

Mix dry ingredients with egg yolks, then add just enough liquid to stir until smooth. Gradually add rest of liquid and stir. Cook on slow heat until thickened. Take off stove and add butter. Stir, cool and add to potato salad. Has a nice sweet-sour taste.