Grease and flour a 10-cup-capacity Bundt pan (or two 8- or 9-inch loaf pans). Preheat oven to 325 degrees. In microwave oven or double boiler over simmering water, melt chocolate. Let cool.
Put espresso and cocoa powders in a 2-cup (or larger) glass measuring cup. Add enough boiling water to come up to the 1 cup measuring line. Mix until powders dissolve. Add whiskey and salt; let cool.
Using an electric mixer, beat 1 cup butter until fluffy. Add sugar and beat until well combined. Beat in the eggs, one at a time, beating well between each addition. Beat in the vanilla extract, baking soda and melted chocolate, scraping down sides of bowl with a rubber spatula.
On low speed, beat in a third of the whiskey mixture. When liquid is absorbed, beat in 1 cup flour. Repeat additions, ending with whiskey mixture. Scrape batter into prepared pan and smooth top. Bake until a cake tester inserted into center of cake comes out clean, about 1 hour 10 minutes for Bundt pan (loaf pans will take less time, start checking them after 55 minutes).
Transfer cake to a rack. Unmold after 15 minutes and sprinkle warm cake with more whiskey. Let cool before serving, garnished with confectioners’ sugar if you like.