Grammy Faith's Mustard Sauce

In the top half of a double boiler or a medium metal bowl stir together the mustard powder white sugar and flour until blended. Gradually stir in the cream so that no lumps form. Place over a pot of simmering water. Cook stirring occasionally until thickened about 15 minutes.

Measure vinegar into a cup and drop the egg yolk in it. Whisk together with a fork then quickly stir into the mustard mixture until well blended. Let stand over simmering water until ready to serve. This sauce can also be reheated over simmering water if there is any left over.