Slowly mix the egg yolk and sugar until the sugar has dissolved.
Boil the milk with the vanilla bean, remove the vanilla bean, and scrape the pulp into the milk.
Slowly pour the hot milk into the yolk mixture, stirring constantly.
Now pour the cream into a pot and mix constantly while heating it (not boiling! ) until the mixture begins to thicken.
You can tell when it is ready because when you blow on the cream while it is coating the cooking spoon, rings will appear that are reminiscent of a rose.