Sprinkle the cornstarch over 1/2 cup of the milk in a small bowl and whisk to dissolve. Add the sugar and egg yolks and whisk well.
In a heavy-bottomed medium saucepan over low heat, bring the remaining 2 cups of the milk and the vanilla bean just to a simmer. Remove the bean, scrape the vanilla seeds into the milk, and discard the bean. Gradually whisk the egg yolk mixture into the milk.
Cook, whisking often, until the sauce comes to a full boil, about 3 minutes. If using vanilla extract, stir it in now. Strain through a wire sieve into a bowl. Serve warm.
Add to the vanilla cream the chopped chocolate chunks and a boiled cup of milk. Mix until smooth. Any flavor can be added- raspberry, cinnamon, holiday ginger spice, marshmallows. The semi-sweet chocolate can be substituted with milk or white chocolate.