Bring the milk, cream, and vanilla bean to a simmer in a saucepan, then remove the pan from the heat and let the bean steep for 10 minutes or longer.
Meanwhile, set a bowl in ice and put a strainer in the bowl.
Remove the bean, scrape out the seeds, and return them to the pot. Put the empty bean pod in your sugar bowl.
In a bowl, whisk together the yolks and sugar thoroughly.
Bring the milk and cream back to a simmer. Pour about half of it into the yolks, whisking continuously, then pour it all back into the pan with the remaining milk and cook over medium heat, stirring continuously with a rubber spatula (or flat-edged wood spoon) until the sauce thickens to nappe consistency (when you lift the spatula out, you can draw a finger through the sauce).
Pour it through the strainer into the bowl set in ice, and stir with the spatula until the sauce has cooled.
Notes
When cooking and stirring the mixture with the egg yolks, make sure to scrape the bottom of the pan with the spatula.