In a heat resistant mixing bowl, stir together the egg yolks and the sugar with a wooden spoon until well combined. The sugar should be somewhat dissolved in the yolks but you don't want to add a lot of air.
Place the milk and cream in a medium sized sauce pan. If you're using a vanilla bean, slit it lengthwise and scrape out the seeds, by pressing firmly along the length with the back of a knife. Add the seeds and the bean pod to the milk. Place the sauce pan on medium heat and warm the milk just below the boiling point (the milk will start to form bubbles on the edges of the pan and it will become steamy just before it boils.)
Remove the sauce pan from the heat and stir just a couple of tablespoons of the hot milk into the eggs and sugar mixing well the whole time. Then gradually add the rest of the milk, stirring the whole time. Return the mixture to the sauce pan and place on low medium heat.
Heat the sauce until it thickens, but do not boil or the yolks will curdle. The sauce is done when it stays on the wooden spoon without dripping (this takes about five minute, but will depend upon your pan and burner). You can run your finger on the back of the wooden spoon and if the track remains without drips, the sauce is done.
As soon as the sauce is thickened, immediately remove from the heat. Pour the sauce into a heat resistant bowl (you can pour the sauce through a strainer to remove the vanilla bean and any clumps). Stir in the vanilla extract if that is what you are using, then cover the sauce with plastic wrap, placing the wrap right on top of the sauce. Refrigerate to desired serving temperature.
Makes 2 cups.
Flavor Variations: Stir in 1 tablespoon of your favorite liqueur after the sauce has cooked. Try Grand Marnier, Cognac, or rum.