In a medium saucepan over medium heat, melt the unsalted butter. Allow it to foam slightly but be careful not to let it brown.
Once the butter is melted, add the packed dark brown sugar and the 1/4 teaspoon of salt. Stir the mixture until the sugar is completely dissolved and the mixture is bubbling gently.
In a separate bowl, whisk together the heavy cream and 2 percent milk. Gradually pour this mixture into the saucepan while continuously whisking to combine.
In a small bowl, mix the cornstarch with the 3 tablespoons of water until smooth. This will prevent lumps in your pudding.
Bring the mixture in the saucepan to a gentle simmer, whisking often. Once it starts to thicken, add the cornstarch mixture, continuing to whisk until the pudding is thick and bubbly, about 2-3 minutes.
Remove the saucepan from the heat and stir in the vanilla extract and, if using, the good scotch. This addition will elevate the flavor beautifully.
Pour the pudding into a heatproof bowl or individual serving dishes. Place plastic wrap directly on the surface of the pudding to avoid a skin forming as it cools. Refrigerate until set, about 2 hours.