Place the milk, cream and vanilla in a saucepan (if you are using vanilla extract you'll whisk it in later).
Set over medium-high heat and bring just to a simmer, stirring occasionally.
Remove from heat.
Meanwhile, place the egg yolks, sugar and salt in a medium mixing bowl and beat with a whisk until well blended.
Remove the vanilla bean from the milk.
Use a knife to scrape out as much of the seed as possible and add them to the pot.
Swirl the vanilla bean in the sauce to be sure you get the rest of the seeds out.
You can use the pod for other uses, like vanilla sugar.
Ladle out about 1/2 cup of the hot milk and whisk it into the egg mixture. (This is called "tempering”).
It allows the eggs to warm up so they don't get shocked into curdling when pour them into the hot milk.
Slowly pour the egg mixture back into the pot of milk, whisking steadily to keep the eggs from curdling.
Place the pot over medium heat and cook the mixture, stirring constantly.
Stir until it coats a spoon about 3-5 minutes, but make sure you don't scramble the egg. (Your finger should leave a distinct trace when you run it along the back of a wooden spoon).
Do not let the mixture boil.
Remove from the heat, strain through a fine-mesh sieve set over a bowl.
Allow to cool slightly. Put in the vanilla extract if you didn't use a vanilla bean.
Cover with plastic wrap and chill in the fridge for at least 2 hours.
It will keep for 4 days.