Preheat oven to 300°F Butter (or spray with Bakers Joy) a 12-cup bundt pan and dust with cocoa powder, knocking out excess.
In a bowl whisk together flour, baking soda, and salt. In a large metal bowl set over a saucepan of barely simmering water melt chocolate and butter, stirring until smooth. Remove chocolate from heat and stir in rum, coffee, and granulated sugar. With an electric mixer beat in flour, 1/2 cup at a time, scraping down side, and beat in eggs and vanilla until batter is combined well. Pour batter into the prepared pan.
Bake cake in middle of oven until a tester comes out clean, about 1 hour and 40 minutes. Let cake cool completely in pan on a rack and turn it out onto rack.
Dust cake with confectioners' sugar and serve with whipped cream.