Put the milk, cream and sugar in a small pan and stir. Split open the half vanilla pod and scrape out the seeds into the cream mixture. Put the pod in as well and warm the mixture over a medium-low heat to bring the mixture almost to a simmer.
Break up the egg yolk in a small bowl and mix in the cornstarch until smooth. Once the cream mixture is almost at a simmer, add a tablespoon of the warm cream to the egg yolk to temper it. Stir in then repeat another two times then tip all of the mixture back in to the cream.
Continue to warm the cream gently, making sure it doesn't get to simmering. Warm until it starts to thicken slightly. Remove from the heat and set aside. You can make this ahead and either warm or serve it cold. If made more than around 1 hour ahead, chill and store in the fridge (can make a day or two ahead of when needed).