Place the halfandhalf in a medium saucepan and bring to a simmer. Whisk together the sugar and egg yolks. Gradually add the halfandhalf to the egg yolk mixture, whisking constantly. Return the mixture to the saucepan and place over mediumlow heat. Cook, stirring constantly with a wooden spoon, until the mixture thickens and leaves a path on the back of the spoon when a finger is drawn across it; about 5 minutes. Do not allow the mixture to boil. Pour the custard through a sieve into a medium bowl. Stir in the vanilla extract. Refrigerate until cold. Transfer the custard to an ice cream maker and process according to manufacturers' instructions. Yield: about 1 quart