Heat the 2 cups whole milk and 2 teaspoons vanilla bean paste in a heavy bottom, medium saucepan until at a low simmer. Do not let it boil, until heat until little bubbles form around the edges.
Meanwhile, place the ⅔ cup sugar, ¼ cup cornstarch and ¼ teaspoon salt into the bowl of a stand mixer fitted with the whisk attachment or a large mixing bowl with a whisk, blend dry ingredients until mixed.
Add the 4 large egg yolks and blend until a pale yellow.
Slowly pour the milk mixture into the eggs while whisking at a low speed. Do not mix too fast or it will froth and foam. Blend until mixed.
Transfer the mixture back to the saucepan and cook over medium-low heat for 3-5 minutes, or until thickened. Whisk continually to prevent mixture from scalding to the pan. Mix until it is thick enough to coat the back of a spoon.
As soon as the mixture thickens, remove from heat and whisk in the 1 tablespoon unsalted butter (or bourbon), if desired.
Transfer to a clean bowl and allow to cool until there is no more steam. Cover with plastic wrap. Press the plastic wrap into the custard mixture so there is no air between the two. This prevents the filmy skin that can develop.
To set faster, place in small ramekins or bowls.
Refrigerate for a minimum of 3 hours, but up to 2 days.
