Over medium/medium-low heat, melt the brown sugar with the salt and the water. Heat until it begins to bubble at the edges, and continue to cook for 5-6 minutes more, keeping a close eye on it and making sure it doesn't burn. Don't stir it while it's cooking, but instead, swirl the pan to distribute the heat if needed.
Take the pan off the heat and allow the melted brown sugar to cool.
Meanwhile, dissolve the cornstarch in a little bit of the milk, and whisk until very smooth. Add the egg yolks, and whisk until smooth.
Add the corn starch slurry to the remainder of the milk and cream, and whisk briefly to combine everything.
Add the milk/cream mixture to the partially cooled brown sugar, whisking as you go. You'll begin to see sugar granules float to the top, but all of it will dissolve once the pan goes back on the heat.
Return the pan to the heat, and gently bring the whole mixture to a boil over medium-low heat, whisking constantly. Once all the brown sugar has dissolved and the mixture is beginning to thicken, switch to a silicone spatula so that you can scrape the bottom and sides of the pan as it thickens.
Once the pudding comes to a boil, (it should be quite thick), take it off the heat and immediately pour it through a fine mesh strainer. You can use the same silicone spatula to push it through the strainer, if needed.
Add the softened butter, vanilla extract, and whiskey to the bowl, and whisk vigorously to combine.
Pour the pudding into serving containers, and enjoy warm, or allow it to fully set in the fridge. It will be fully set in a couple hours. (we prefer it chilled!)