Heat milk and vanilla extract in a heavy bottom, medium saucepan until at a low simmer. Remove a small amount of milk and whisk with corn starch. Do not add cornstarch directly to milk or it will clump. Add cornstarch slurry back to milk and whisk in salt.
Meanwhile, place egg yolks and sugar into the bowl of a stand mixer fitted with the whisk attachment or a large mixing bowl with a whisk ready to use. Blend until a pale yellow.
Slowly pour milk mixture into eggs while whisking at a low speed. Do not mix too fast or it will froth and foam. Blend until mixed.
Transfer back to the saucepan and cook over medium-low heat for 2-4 minutes, or until thickened. Whisk continually to prevent mixture from scalding to the pan.
As soon as mixture thickens, remove from heat and whisk in butter, if desired.
Transfer to a clean bowl and cover with plastic wrap. Press 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.
If you have tried this recipe, come back and let me know how it was!
To use a whole vanilla bean, cut open and use 1/2 teaspoon vanilla caviar (little specs inside) in the scalded milk. You can also use a leftover, spent vanilla bean in the milk and remove before adding to egg yolks. Do not use skim or 2% milk, this really does require WHOLE MILK.