Beat egg yolks and add sugar. Beat well. Make a paste of cornstarch and 1/4 cup milk. Combine egg mixture with remaining milk and cornstarch mixture. Cook, stirring constantly, in a double boiler over barely simmering water until desired consistency is reached. (When the mixture coats the back of a spoon, it’s done.)
Let cool completely. Pour into a pitcher and stir in vanilla. Refrigerate, uncovered, until cold. (Mixture will thicken as it cools.) Once cold, cover, refrigerated up to 5 days Stir well before serving. Makes 4 cups.