Scald the milk. Beat yolks, sugar, and salt together in small mixer bowl. Gradually beat in the hot milk, then pour back into the pan the milk was in.
Set pan on an asbestos mat and cook over direct heat, or cook the sauce in a double boiler. It must not get too hot, and it must not boil. Stir-cook over very low heat until the sauce thickens. The foam on top