In a 3- to 4-quart pan over medium heat, cook milk, cream, and sugar, stirring often, until mixture begins to steam and bubble around edges of pan, about 8 minutes. Remove from heat.
Put egg yolks in a bowl and whisk in about 1/2 cup hot milk mixture, then pour egg mixture back into pan and whisk to blend. Cook, stirring with a flexible, heatproof spatula over medium-low heat until custard reaches 160° on an instant-read thermometer and thickens noticeably, 5 to 8 minutes. Remove from heat. Add chocolate and whisk until melted and smooth. Stir in vanilla.
Pour through a fine strainer set over a bowl. Set the bowl of custard in a larger bowl of ice and water for 10 minutes, stirring frequently, until mixture cools. Cover with a layer of plastic wrap pressed against surface of custard. Chill at least 1 hour and up to 3 days.
Other crème anglaise uses. Use this versatile custard in any number of desserts. Some of our favorite options:
In a tart: Pour into a baked pastry shell and top with raspberries.
In a layer cake: Spread the sauce between each layer, then frost with your favorite icing.
In chocolate cream pie: Pour into a prepared cookie-crumb crust and top with whipped cream and chocolate shavings.
In a parfait: Layer the sauce with vanilla crème anglaise (or vanilla pudding), almonds, and crushed meringues or shortbread cookies.
As a pudding: Simply serve as is, with a bit of whipped cream.
Note: Nutritional analysis is per 1/2 cup serving.