Take 1-2 tbsp of milk from the original pint of milk and add it to the cornstarch. Mix together to make a slurry. Make sure there are no lumps.
Put the egg yolk, sugar and the remainder of the milk into a pot. Warm this mixture over a low to medium heat.
On a chopping board and with a firm grip on the vanilla bean, slice it lengthways. Lay each half length flat on the board and scrape out the pods using a paring knife. Transfer these pods to the pot.
Add the cornstarch slurry to the pot. The pot contents will start to thicken as the sauce forms. Keep stirring gently and continuously. Simmer for a minute and then remove from the heat.
Notes
German Vanilla Sauce has a thinner consistency than most sauces. It will thicken a little as it cools, but it is not meant to be a thick sauce.Serve the Vanilla Sauce warm or at room temperature.