Put the milk in a medium saucepan over a low heat. Scrape the vanilla seeds into the milk, using a knife. Whisk, then throw in the vanilla pod as well. Simmer for at least 15 minutes to get the maximum flavour out of the seeds. Meanwhile, whisk the egg yolks and sugar together in a large bowl.
Add the egg-yolk mixture to the milk and cook over a medium heat for 5–8 minutes, stirring con