Crème Anglaise (or "English cream") is a classic French custard sauce that complements just about any cake, but it's divine with chocolate desserts. It will keep, covered, in the refrigerator for 3 days.
Combine the cream and vanilla bean in a medium-size heavy-bottomed saucepan. If you’re not using the vanilla bean, wait to add the vanilla extract until the sauce has been cooked.
Place the pan over medium heat and bring the cream to a simmer. Then turn off the heat and let the mixture steep for 10 minutes. Remove the vanilla bean. Using a small knife, scrape the seeds into the cream. Wash the pod and reserve for another use.
Whisk the egg yolks and sugar in a medium-size bowl. Gradually whisk in the warm cream. Return the egg-cream mixture to the saucepan and place over low heat. Cook the custard, stirring or whisking constantly, until it thickens enough to coat a spoon, 7 to 10 minutes. Don’t let the custard boil or the eggs will scramble. Small wisps of steam will begin to rise from the custard and it will feel very warm to the touch when it’s ready to be removed from the heat.
Remove the custard from the heat and add the vanilla extract, if you’re using that instead of a vanilla bean. Transfer the crème Anglaise to a clean bowl and refrigerate until cold. Cover with plastic wrap and keep refrigerated until serving time.