1In the top of a double boiler, scald milk with vanilla bean. If you use extract, scald milk, then stir extract in.
2Beat egg yolks with sugar and cornstarch until light and frothy. Pour vanilla-flavoured milk into egg yolk mixture slowly, stirring vigorously.
3Pour mixture back into top of double boiler and set over hot (not boiling) water that does not touch the upper pan.
Cook over simmering water, stirring constantly, until sauce becomes custardy and thick enough to coat a wooden spoon.
4If a foamy sauce is preferred, beat egg whites into hot custard until frothy. Serve hot or cold.