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. Heat the milk until it is almost simmering, remove from the heat and leave to infuse for about 20 minutes. Remove the vanilla pod from the milk and clean and dry in kitchen towel – you can use it again.
Meanwhile, whisk the egg yolks tog