Grease a 10-cup-capacity Bundt pan, and dust with superfine/caster sugar.
Preheat oven to 325 degrees F.
In microwave oven or double boiler over simmering water, melt chocolate. Let cool.
Put espresso and cocoa powders in a 2-cup (or larger) heatproof glass measuring cup.
Add enough simmering water to come up to the 1 cup measuring line. Whisk until powders dissolve.
Add whiskey and salt; let cool.
In a large bowl, use an electric mixer to beat softened 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 lowest speed, beat in a third of the whiskey mixture, dribbling it in slowly to avoid cooking the eggs.
When liquid is absorbed, beat in half of the flour.
Repeat additions, ending with whiskey mixture.
Pour batter into prepared pan, banging the pan against the counter to get rid of the largest bubbles.
Bake until a cake tester inserted into center of cake comes out clean, about 1 hour 10 minutes.
Transfer cake pan to a cooling rack.
Unmold after 15 minutes, optionally garnishing with confectioners' sugar before serving.