Mix all ingredients in a medium heatproof bowl. Set it over a pot of simmering water. Cook, stirring constantly, until the sauce thickens slightly, 5 to 7 minutes. A finger drawn across the back of the spoon should leave a trail. Do not overcook the sauce since it will curdle. Taste and adjust the seasoning. Sauce can be kept warm over a pan of hot water, but should be used as soon as possible.