Put the yolks in a saucepan and add the sugar. Beat with a wire whisk until thick and lemon colored.
Put the cream in a nonreactive saucepan and heat to the scalding point (when bubbles form around the edge of the pan). Gradually add the cream to the yolk mixture, beating constantly. Cook over low heat until the mixture thickens slightly. Do not overcook or boil, as the sauce will curdle. Remove from the heat and stir in the vanilla.
Strain through a fine-mesh sieve into a cold bowl. Cover with plastic wrap, pressing it down on the surface to prevent a skin from forming if not using immediately. Will keep refrigerated for 24 hours. Before serving, warm over low heat.
Yield: about 2 1/2 cups