Split open the vanilla bean and place in a small pot along with the milk. Heat over medium low heat, meanwhile, prepare the rest of the custard.

In a small bowl, whisk the yolks, sugar and corn starch together until well combined.

When the milk begins to simmer, remove the vanilla bean and pour about half a cup of hot milk into the egg mixture, whisking quickly. Add the rest of the milk, then whisk and return to the pot.

Heat over medium heat and continue stirring, taking care that the custard doesn't stick or burn, using a wooden spoon is best at this point (the photo below is before I switched to the wooden spoon)! Add the vanilla extract if not using the vanilla bean. Continue to stir until the custard begins to thicken.

Remove from heat, use as desired. If pouring over a dessert, use immediately.