1) Sprinkle the cornflour over 120ml of the milk in a small bowl and whisk to dissolve. Add the sugar and egg yolks and whisk well.
2) In a heavy-bottomed medium saucepan over low heat, bring the remaining 480ml of the milk and the vanilla pod just to a simmer. Remove the pod, scrape the vanilla seeds into the milk and discard the husk. Gradually whisk the egg yolk mixture into the milk.
3) Cook, whisking often, until the sauce comes to a full boil, about 3 minutes. If using vanilla essence, stir it in now. Strain through a wire sieve into a bowl.
4) Add to the vanilla cream the chopped chocolate chunks and a boiled cup of milk. Mix until smooth. Any flavour can be added, for example, raspberry, cinnamon, holiday ginger spice, marshmallows. The dark chocolate can be substituted with milk or white chocolate.