Crème Anglaise

In a 1 1/2-quart heavy saucepan bring half-and-half and heavy cream just to a boil. While cream mixture is heating, in a bowl whisk together yolks, sugar, and cornstarch until slightly thickened and pale, about 2 minutes. Add hot cream mixture in a stream, whisking, and transfer custard to pan. Cook custard over moderately low heat, stirring constantly with a wooden spoon, until a thermometer registers 180°F., about 2 minutes (do not let custard boil). Transfer custard to cleaned bowl and cool to warm, stirring occasionally. Crème anglaise may be made 2 days ahead and chilled, covered. Reheat sauce if desired.