Preheat the oven to 350°F., To make the cake: Place the coffee, butter, and cocoa in a small saucepan or microwave-safe bowl. Heat, stirring, until the butter melts. Remove from the heat, and whisk until smooth. Let the mixture cool for 10 minutes., While the chocolate is cooling, put the sugar, baking powder, baking soda, salt, and flour into a mixing bowl, whisking to combine., Pour the cooled chocolate mixture into the bowl with the dry ingredients, and mix until thoroughly combined. Scrape the bottom and sides of the bowl, and mix again to incorporate any residue., In a separate bowl, whisk together the vanilla, eggs, and sour cream or yogurt. Mix into the chocolate batter, stirring until thoroughly combined., Thoroughly grease a 10- to 12-cup Bundt pan, preferably non-stick. Pour the batter into the prepared pan. Bake the cake for 50 to 55 minutes, until a long toothpick or skewer inserted into the center comes out clean. Note: a pan with a dark interior will bake the cake more quickly; start checking at about 40 minutes. , Remove the cake from the oven, wait 5 minutes, and turn the pan over onto a cooling rack. After 5 more minutes, lift the pan off the cake. Let the cake cool completely before icing., To make the icing: Combine the chocolate and cream in a microwave-safe bowl, or in a saucepan set over medium heat. Heat until the cream starts to bubble around the edges., Remove from the heat, and stir until the chocolate melts and the mixture is smooth. Spoon the icing over the top of the cake, letting it drip down the sides., Store any leftover cake at room temperature, well wrapped, for several days. Freeze for longer storage.