Pour the milk and cream into a heavy-bottomed pan over a gentle heat and bring slowly to a simmer.
Cut the vanilla pod in half with a small sharp knife and scrape out the seeds. Add the vanilla seeds and the pod to the milk and cream mixture to infuse.
Meanwhile, separate the egg yolks and egg whites. Reserve the egg whites for use in another recipe of your choice, such as meringues.
Add the sugar to the yolks and beat together using a balloon whisk until the sugar is incorporated into the yolks.
When the milk and cream is just simmering, pour three-quarters of it onto the sugar and egg yolks and whisk to combine.
Pour this mixture into the remaining milk and cream in the pan and return to the heat, gently stirring all the time. Don't let the mixture boil.
Cook slowly, stirring continuously, until the mixtures thickens to the consistency of thick double cream.
When it has reached the desired consistency, remove from the heat and pour the mixture through a sieve to remove the vanilla pod and any small lumps that may have formed.
Serve warm or allow to cool - the mixture will thicken slightly as it cools.