Stir starch with a little milk until smooth. Put the remaining milk, sugar and salt in a pot. Cut the vanilla pod lengthwise and scrape the pulp into the milk. Add the pod to the milk and bring everything to the boil. Stir in cornflour and let it boil for about 1/2 minute.
Take the sauce off the stove. Mix some of the hot sauce with the egg yolks, then stir the egg yolks into the hot sauce. Remove the vanilla pod. Vanilla sauce tastes hot and cold with fruit, groats and sweet baked goods (e.g. apple strudel)