In a bowl beat together the yolks and the sugar until mixture is light and forms a ribbon when the beater is lifted. Add the half-and-half, scalded, in a stream, stirring, transfer the custard to a heavy saucepan, and cook it over moderately low heat, stirring, until it thickens, but do not let it boil. Remove the pan from the heat and stir in the vanilla. Strain the custard through a fine sieve into a metal bowl set in a bowl of ice, let it cool, stirring occasionally, and chill it, covered. Makes about 3 cups. Gourmet May 1993