Preheat oven to 350F/180C. Grease a 12-cup (10-inch) Bundt pan and coat with cocoa powder, tapping the pan slightly to remove any excess. Set aside.
In a medium heatproof bowl, place cocoa and coffee powder. Pour hot water over the mixture and let stand for 1 minute, then whisk until smooth. Let the mixture cool to room temperature (meanwhile, proceed with the rest of the recipe), then whisk in the sour cream. In another medium bowl, sift flour, baking soda, and salt, and mix to combine. Set aside.
In a mixer bowl fitted with the paddle attachment, beat together butter and sugars on medium-high speed until light and fluffy, about 3 minutes. Add eggs, one at a time, beating well after each addition. Scrape down the sides and bottom of the bowl as necessary. Beat in vanilla extract. On low speed, beat in the flour mixture in 3 additions, alternating it with the cocoa-sour cream mixture in 2 additions, starting and ending with the flour. Do not over mix.
Scrape batter into the prepared pan and smooth the top. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Allow cake to cool in the pan for 15 minutes, then gently invert onto a wire rack and let it cool completely. Dust with powdered sugar or drizzle with glaze.
To make the chocolate glaze: Place chocolate in a medium heatproof bowl. Set aside. In a small saucepan over medium heat, bring heavy cream just to a boil. Immediately pour cream over chocolate and let sit without stirring for 2 minutes. Then, whisk mixture gently until completely melted and smooth. Let cool for 10 minutes before using. Alternately, heat chocolate and cream in the microwave in 20-second intervals, mixing between each, until melted.