Place the split vanilla bean and the milk in a small sauce pan and bring to a simmer, remove from the stove and let the vanilla bean infuse with the milk for 10 minutes.
Meanwhile in a small bowl place the egg yolks and the sugar and beat with a whisk or a hand held electric mixer until the sugar and eggs double in volume and become pale in color. If you are using vanilla extract add it at this time. This should take about 4-5 minutes.After the milk has sat for 10 minutes infusing, place it back on the heat and heat until it is just ready to simmer.
Now pour the milk into the egg mixer 1/3 of it at a time, stirring it all the time to be sure not to scramble the eggs. When all the milk has been incorporated pour it back into the sauce pan through a sieve to remove the vanilla bean and any bits of cooked egg white that you might have in the mixture and place on the stove over a low heat.
Now heat the milk and eggs to thicken the yolks. You will have to stir the entire time but this should only take a few minutes. Don't take your eye off of it for a moment.
Slowly bring the mixture up in heat until it is thickens and coats the back of a spoon.
The mixture must never boil.
When thickened remove from the heat and place the pan into a bowl of very cold water to stop the cooking. Place the pan into another bowl of cold water with ice to chill as quickly as possible.
Whisk the sauce occasionally while the sauce is cooling.