Butter a 10-cup bundt pan. Set aside. Preheat oven to 325 degrees F
In a microwave safe bowl, heat unsweetened chocolate at half power at 30 second intervals. Stir often. Heat until chocolate is melted. Set aside.
In a glass measuring cup, stir together espresso power and cocoa powder. Add enough hot water until liquid mixture is 1 cup. Stir until powders are dissolved. Add one cup of whiskey and let cool.
In the bowl of a stand mixer with a paddle attachment, cream butter and sugar until light and fluffy. Add eggs one at a time, making sure egg is incorporated before adding the next. Scrape down bowl as needed. Add vanilla and slightly cooled melted chocolate. Continue to mix on low-medium until combined.
In a medium bowl, whisk together flour, baking soda, and sea salt. With the mixer on low, add 1/3 of whiskey mixture, then 1/2 of flour mixture. Repeat, ending with whiskey mixture. Mix until just combined.
Transfer batter to prepared bundt pan. Bake for 70 minutes, rotating halfway, until toothpick inserted in center of cake comes out clean. Let cool in pan for a couple minutes before unmolding. Let cool on wire rack until room temperature.