In a small heavy saucepan bring half-and-half just to a boil with vanilla bean and remove pan from heat. Scrape seeds from vanilla bean with a knife into half-and-half, reserving pod for another use. While half-and-half mixture is heating, in a bowl whisk together yolks and sugar. Add hot half-and-half mixture in a stream, whisking, and transfer to a saucepan. Cook custard over moderately low heat, stirring constantly with a wooden spoon, until thickened (170 F. on a candy thermometer), but do not let boil, and strain through a fine sieve into a metal bowl. Set bowl in a larger bowl of ice and cold water and cool sauce, stirring occasionally. Chill sauce, covered, at least 2 hours, or until very cold. Makes about 2 cups.