Bring about 3 inches of water to a simmer in the bottom of a double boiler. Whisk together the sugar, vinegar, tomato soup, mustard powder, egg yolks, and melted butter in the top of the double boiler. Transfer 1/4 cup of the mixture to a small bowl; mix in flour until smooth. Whisk flour mixture back into mustard sauce; combine thoroughly. Cover and turn heat to medium-high.
Cook sauce until thick, approximately 30 minutes; stir occasionally. Turn heat to low and simmer sauce to hold.
