Set aside 1 tablespoon of milk. Slowly bring the rest of the milk and sugar to the boil while stirring. Slit the vanilla pod lengthways, scrape out the pulp and add the pod to the milk. Mix the cornstarch with the rest of the milk until smooth, add to the boiling milk and bring to the boil briefly. Take the sauce off the stove. Remove the vanilla pod, stir in the egg yolks. Let the sauce cool down, stirring occasionally.