Heat the milk with the crème fraîche the vanilla seeds, and the bean until hot (do not boil) and leave to simmer on low heat for 15 minutes. Then remove the vanilla bean. Beat the egg yolks with the sugar and cornstarch into a foam, and while stirring, add the hot milk. Return the mixture to the pan. Heat slowly on low heat while stirring, until the mixture thickens. Remove from the heat and leave to cool until used. The custard will further thicken. If you want to serve the custard warm, keep it over a double boiler. Place a sheet of plastic wrap directly down on the custard to prevent a skin from forming.