Mix the egg yolks, sugar and cornstarch in a bowl. Whisk until sugar has dissolved (and the mixture has become lighter in color).
Pour the milk and cream into a saucepan. Add the vanilla pod. Bring to a simmer over medium heat.
Pour 1/3 of the hot milk mixture into the eggs. Vigorously whisk the eggs so that the hot milk doesn't cook them!
Pour the eggs mixture into the pot with the rest of the milk. Vigorously whisk until combined.
Place the pot back on the heat. Heat while stirring continuously with the whisk until the mixture begins to thicken. Continue to cook for up to another 5 minutes.
Remove the pot from the heat and continue to stir for another 2 minutes. Transfer the pastry cream into a bowl and remove the vanilla pod. Let cool at room temperature. Once at room temperature, refrigerate.
Thin the pastry cream as desired by stirring with a whisk and/or adding a bit of cream or whole milk.