In a heavy saucepan, combine the milk and heavy cream. Scrape the seeds from the vanila bean. Cook over medium heat and bring to a simmer.
Once the sauce begins to simmer, remove the pan from the heat, add the sugar and stir to dissolve. Let it cool for 30 minutes. Discard the vanilla pod.
In a separate small bowl, whisk the egg yolks until blended.
Return the milk mixture to a boil over medium heat and then remove from the heat.
Add a 1/4 of the milk mixture to the egg mixture and stir quickly. Then the yolk mixture into the pan whisking until blended.
Return the mixture to medium heat and cook stirring constantly until the sauce is thick enough to coat the spoon (3-4 minutes). DO NOT LET THIS BOIL.
Transfer to a heat proof bowl and stirl frequently until it is room temperature. If you are using vanilla extract add it now. Stir until it is all mixed.
Cover and refrigerate until you are ready to use. This will keep for up to 2 days.