Prepare an 8x8 pan with parchment and a nonstick baking spray.
In a bowl on top of a pot of simmering water (double boiler)
Heat the chocolate until fully melted and smooth.
Add in the sweetened condensed milk and butter and continue heating for 2 minutes. It will be thick when you stir it and smooth, the butter will be completely melted and the condensed milk will be all incorporated as you stir it.
Remove from heat.
Add in the salt, vanilla extract, and bourbon or whisky and mix until all incorporated and smooth.
If you want nuts added to the fudge stir them in now.
Pour into prepared pan, smooth the top with an offset spatula and immediately top with salt or nuts.
Tap the pan on the counter gently once or twice to remove any air bubbles is necessary.
Place the pan in the refrigerate to completely chill and set the fudge for 2 hours.
When the fudge is completely cold, remove it from the pan by lifting the parchment paper out so the fudge removes easily.
Slice the fudge with a smooth knife. (running the knife under warm water then drying it will allow the fudge to slice smoothly)
Fudge is best served at room temperature (out of the fridge for 30 minutes) for best flavor and texture.