Vanilla Sauce

Vanilla Sauce:
1 cup whole milk
1 cup whipping (heavy) cream
½ vanilla bean, split and scraped Or, 2 teaspoons pure vanilla extract
4 large egg yolks
⅓ cup granulated sugar
1/16 teaspoon salt

Vanilla Sauce:
In a medium, non-reactive, heavy-bottomed pan over medium heat, combine the whole milk, whipping cream, vanilla bean, and scraped vanilla bean seeds. (If using vanilla extract in place of the vanilla bean do not add to the milk; instead the vanilla extract will be added after the Vanilla Sauce has been cooked.) Bring the milk mixture to a simmer (do not boil) stirring constantly with a rubber spatula or wooden spoon, remove from the heat.
Meanwhile, while the milk mixture is heating, in a medium bowl with a wire whisk, whisk the egg yolks, sugar, and salt together.
Temper the eggs: Very slowly, in a thin stream, pour about ½ cup of the hot milk mixture into the beaten eggs while quickly whisking the two together with a wire whisk. Return the egg mixture back to the rest of the hot milk, pouring back slowly while quickly whisking the two mixtures together. Return to medium-low heat, stirring constantly with a rubber spatula in a figure eights pattern around the edge of the pan and into the center, making sure to scrape the rubber spatula across the bottom of the pan. Continue to cook while stirring constantly about 2 minutes or until the mixture registers 170 to 175 degrees F. but no more than 180 degrees F. using a candy or instant read thermometer to gauge the temperature. Do not allow mixture to boil or the egg yolks will curdle. Remove from heat. Tip: A heat-resistant rubber spatula is best for stirring because it can scrape the entire bottom of the pot to help prevent the egg from clumping together and coagulating.
Finishing the Vanilla Sauce: Remove and discard the vanilla bean if using. Or, if using vanilla extract, add it now and stir until completely blended. Let cool to room temperature, stirring frequently while cooling. Or, Pour the Vanilla Sauce into a medium-sized bowl and place a piece of plastic wrap directly on the surface while the sauce is cooling to prevent a skin from forming. The Vanilla Sauce will thicken as it cools.
The Vanilla Sauce can be used immediately or cooled to room temperature and refrigerated for up to 2 days.
Source: Peterson, James, Baking, Ten Speed Press, New York, 2009; Greenspan, Dorie, Baking, Houghton Mifflin Company, New York, 2006; Yard, Sherry, The Secrets of Baking, Houghton Mifflin Company, New York, 2003