Preheat oven to 350 F. Spray a 10-inch bundt pan with nonstick cooking spray and sprinkle the inside with flour.
Make the Cake:Combine brewed coffee and cocoa powder in a small saucepan and bring to a boil, whisking frequently. Remove from the heat and let cool back down to room temperature, about 30 minutes.
When the the coffee mixture is nearly cool, combine sugar, salt, baking soda, eggs and egg yolk in the bowl of a standing mixer fitted with the whisk attachment. Mix on low until evenly combined, 1 minute.
Add buttermilk, oil, and vanilla extract and mix on low for 1 minute. Add flour and mix on medium for 2 minutes. Add cooled coffee mixture and mix on medium for 3 minutes. (Note: The batter will be very runny and should pour easily.)
Pour into bundt pan and bake until a knife inserted in the middle comes out clean, 60 minutes.
Let the cake cool completely before flipping it out onto a cake stand. At this point you can keep the cake at room temperature or refrigerate it (covered with plastic wrap) for 1 day.
Make the Glaze: Put chopped chocolate in a heatproof bowl a over a pot of barely simmering water (make sure the bottom of the bowl is not touching the water directly; or use a double broiler). Heat, stirring frequently, until fully melted.
In another bowl, melt the butter (I do this in the microwave in 15 second increments, stirring between each).
Whisk melted butter into melted chocolate until fully incorporated. Sift in half the powdered sugar. Add the sour cream and whisk to combine.
Sift in the remaining powdered sugar and whisk until smooth (you'll have to use some elbow grease here!). When fully mixed, the glaze should be thick and shiny.
Add coffee and whisk again to create a glossy glaze. (A couple times, I've had to add a splash of additional hot water to get the glaze smooth and creamy.)
Pour the glaze over the surface of the cake, letting it drip down the sides and middle.
Let the glaze set for at least 1 hour before serving.