Ham Sauce

In a medium bowl whisk together the sugar mustard powder and flour. Mix in egg yolks until smooth. Heat half and half in the top half of a double boiler or in a metal bowl over a pan of simmering water until just short of boiling. Stir in the egg yolk and sugar mixture and then stir in the vinegar. Continue to stir over simmering water until thickened about 5 minutes.