Pour the milk into a pan and bring to boil. Add the vanilla pod, then remove from the heat and leave to infuse for 30 minutes.
Whisk the egg yolks with the sugar for 2–3 minutes or until pale. Sift in the flour and cornflour, and whisk to combine. Remove the vanilla pod/essence from the milk and scrape the seeds into the milk with a sharp knife; discard the pod. Bring the milk just to the boil, then gradually whisk it into the egg yolk mixture.
Pour the mixture back into the pan and bring to a boil, stirring constantly with a wooden spoon or a whisk. When thick, simmer gently for 1 minute, still stirring constantly. Remove from the heat. Cover the surface of the pastry cream with wet wax paper and leave to cool.