Place the milk in a saucepan, and heat over medium high heat.
While heating the milk to a simmer , place the sugar, egg yolks, cornstarch, vanilla extract, and salt in a large bowl. Whisk until you have a thick, smooth mix and set aside.
As soon as the milk starts to simmer, remove it from the heat. Slowly pour about a ½ cup of the hot milk into the egg mix, while whisking constantly. Once the egg mixture has been tempered completely add it back to the sauce pan.
Heat the custard base, over medium heat, while whisking vigorously until it starts to thicken after a few minutes.
Once the custard releases a bubble to the surface, lower the heat and cook for another 4 minuted while whisking constantly.
Remove from the heat and add the butter. Stir until completely mixed in
Pour the custard through a sieve to remove all lumps and pour into into a chilled bowl. Cover the surface with plastic wrap, making sure the plastic wrap is touching the whole surface. This is to prevent a custard skin from forming on top.
Let the custard cool down to room temperature and then let it chill in the fridge until completely chilled.