For the cake: Preheat the oven to 350 degrees. Grease and flour a 10-12 cup capacity bundt pan with butter or use Baker's Joy spray. Set aside.
Place cocoa powder and coffee in a small saucepan and bring to a boil, whisking constantly. Remove from heat and allow to cool.
In the bowl of a mixer, combine sugar, salt, baking soda, eggs, and yolk. Beat on low speed for about 1 minute. Add buttermilk, oil, and vanilla and beat for another minute or so.
Add flour to sugar mixture and beat on low speed for 30 seconds, then medium speed for about 2 minutes, scraping the bowl once. Add the cooled cocoa/coffee mixture and beat on low speed for another minute. Scrape the bowl to make sure everything is well mixed.
Pour into the greased pan and bake for about 1 hour, or until a toothpick inserted in center comes out clean. Let the cake cool at least 15 minutes before inverting onto serving tray.
For the frosting: Chop the chocolate into small pieces and add it to a double boiler or heat proof bowl over a pot of simmering water. Melt chocolate, stirring occasionally.
In a small bowl, whisk together the butter and chocolate. Add half the powdered sugar and whisk again. The mixture will seem very thick.
Whisk in sour cream and the remaining powdered sugar. Don't worry! It will come together! And it will be magical.
Add coffee and cream and whisk together until smooth and glossy. This is a thick, spreadable frosting. If you'd like a more pourable glaze, simply add additional cream or half and half until you reach your desired consistency.
Once the cake has cooled, spoon the frosting over the cake, gently prodding it to run down the sides. You might have extra frosting! Save it for individuals to add to their servings or use it as a fruit dip!