In a medium bowl, stir together the powdered sugar and whisky with a wooden spoon (you'll need its sturdiness later). If the mixture seems too thick, add about 1 tablespoon of water to loosen. Set aside.
Melt the dark chocolate with the butter in a microwave-safe bowl on HIGH for about 1 to 1½ minutes, stirring after each 30 second intervals. Alternatively, in a small saucepan, heat chocolate and butter on medium-low heat until they're melted, stirring frequently to keep the mixture from burning.
When the chocolate mixture has melted completely, pour it into the whisky mixture and stir vigorously until completely incorporated. It will take some arm work as the mixture gets pretty thick. Stir in the nuts, if using.
Working quickly, transfer the fudge into the prepared pan and smooth to the edges by covering the surface with plastic wrap and pressing the fudge with your hands or a pie pan roller.
Let the fudge sit at room temperature until it's hard enough to cut, about 15 to 20 minutes. You can speed up the process by putting the pan in the refrigerator and checking the fudge’s consistency every 5 minutes or so.
Once the fudge has set, immediately cut it into 1-x 1-inch pieces. Don't let it sit too long before cutting or be more difficult to cut.
Store in an airtight container at room temperature, separating layers of fudge with waxed paper so that they don’t stick together.
Use a good quality whisky. It will be tasted! I used Kavalan Distillery Select, but any good quality whisk(e)y or bourbon will do. If you want to reduce the amount of alcohol, I recommend replacing that amount with another flavorful liquid like milk or apple cider and about 1½ to 2 teaspoons of vanilla.While you’re at it, use a good quality of dark chocolate also. It’s going to make or break the fudge’s texture and underlying taste. I use a 72% dark chocolate, but you can used whatever type of chocolate you'd like. Semi-sweet or milk chocolate would work well.Whisky fudge is served best at room temperature, kept in an air-tight container. It can also be wrapped in wax paper and frozen in an air-tight bag for a month or two.The ingredients can be doubled and pressed into a 9- x 13-inch baking pan. Cutting the fudge into 1- x 2-inch bars will yield 48 pieces.