Add the vanilla pod and half the sugar to the milk mixture and bring it to a simmer. Scrape the vanilla seeds into the milk mixture.
Take it off the heat and let it cool as you attend to the eggs.
Whisk the egg yolks in a mixing bowl with the rest of the sugar and cornflour until pale looking.
Very slowly, add the milk to the egg yolks, whisking all the time.
Transfer everything back into the saucepan and heat it very gently, until it thickens, about 5 minutes.
Notes
If not using immediately, cover with clingfilm, letting the film touch the surface of the custard, cool then refrigerate.I have made this up to 2 days in advance. Reheat on low heat or in microwave.