Heat cream and vanilla in a small heavy saucepan over medium heat until bubbles form at the edges.

While the cream mixture is heating, whisk together egg yolks and sugar in a large bowl until smooth.

Slowly pour 1/2 cup of hot cream mixture into the egg yolk mixture, whisking constantly. Gradually add the egg yolk mixture back to the remaining cream mixture in the saucepan, whisking constantly. Continue to cook, stirring constantly, until the mixture coats the back of a spoon.