In a sturdy 2-qt saucepan, add Brown Sugar, Kosher Salt, and Water. Whisk to combine. Place over medium-high heat. Insert a candy thermometer. Do not stir or disturb.
Meanwhile in a large mixing bowl, whisk together egg yolks and cornstarch until combined.
When the sugar mixture reaches about 235-240 degrees, pour in the Milk and whisk to combine. Reduce heat to medium.
Once the sugar mixture has come to a boil, remove the pan from heat. Begin tempering the eggs. The way this is done: pour a small amount of the hot milk into the egg mixture while whisking rapidly. Then whisk in a little more. Continue adding the milk mixture slowly while whisking until the entire milk mixture is combined with the eggs.
Immediately pour the entire mixture back into the saucepan and put it back on the stove at medium heat. Whisk constantly until the pudding has thickened.
Turn off the heat. Add the Butter, Whiskey, and Vanilla Extract. Whisk until combined. Work the pudding through a mesh strainer or sieve while it’s still hot. Use the back of a spoon to push the pudding through the mesh, leaving only the solid bits behind. Cool the entire pudding in the refrigerator until serving.
Do not freeze. The pudding keeps nicely for 3-4 days in the refrigerator. It is best to store with a piece of plastic wrap pressed directly against the surface of the pudding. This prevents a skin from forming.