Preheat oven to 325F and position the oven rack in the middle of the oven. Grease a 9 inch baking pan with butter or spray and coat the inside of the pan with flour. Remove any excess flour.
Place the chocolate in the bowl of a double boiler and melt the chocolate. Once melted, set aside to slightly cool.
In a large bowl of a stand mixer fitted with a paddle attachment, add the butter and sugar. Cream the mixture on a medium speed until pale yellow and thick, about 4 minutes. Scraping down the sides halfway through.
While the butter is mixing, sift the flour, cocoa powder, baking powder, baking soda and kosher salt into a medium bowl.
Reduce the mixer speed to low and add the eggs, one at a time, until well incorporated. Slowly add the melted chocolate to the mixture and mix until well incorporated.
With the mixer still at a low speed, slowly add the flour mixture and mix until combined, then slowly add the espresso and whiskey.
Remove the bowl from the stand mixer and, using a silicone spatula, scrape down the entire outside and bottom of the bowl, to make sure everything is incorporated.
Pour the batter into the prepared pan, and bake in the preheated oven until an inserted cake tester is clean when removed, about 45 minutes.
Remove the cake from the oven and cool in the pan for 5 minutes. Invert onto a cooling rack and cool at least at least 10 minutes or cool completely and wrap tightly in plastic wrap until ready to serve.
To serve: Lightly sprinkle the chocolate cake with confectioners’ sugar and serve with whipped cream.