Add the milk and cream and the vanilla pod (not extract yet) to a large saucepan and bring to a simmer slowly. once simmering take out the pod.
Mix the yolks, cornflour and sugar in a large bowl until they are well mixed. into this slowly add the simmering milk and cream mix making sure to mix well all the time.
Return all the mix back to the pan (add extract now if using that rather than pods) and use a wooden spoon to mix over a low heat until the custard begins to thicken, check to see if it is coating the back of the spoon to do this.
You can either serve right away or it can be stored in the fridge and warmed later but you will want to cover the top with something so that you don't get a skin over the top. This can be eaten warm or cold.