Heat the oven to 350°F with a rack in the middle position. To prep the pan, in a small bowl, stir together the butter and cocoa. Using a pastry brush, coat the inside of a 12-cup nonstick Bundt pan with the mixture, including the center tube; make sure all bumps and crevices are coated.

To make the cake, in a large bowl, whisk together the sugar, flour, cocoa, baking powder, baking soda and salt; add the coffee and whisk until no lumps remain. Add the eggs, buttermilk, oil and vanilla, then gently whisk until well combined and the batter is smooth.

Pour the batter into the prepared pan, then gently rap the pan several times against the counter to release any air bubbles. Bake until a toothpick inserted into the cake about 2 inches from the edge comes out clean, 40 to 45 minutes. Cool in the pan for about 20 minutes, then invert the cake onto a wire rack and cool to room temperature. Transfer to a platter.

To make the glaze, in a medium saucepan over medium, bring 1 inch of water to a bare simmer. Put the butter, cream, sugar and salt into a heatproof medium bowl and set the bowl on top of the saucepan; make sure the bottom does not touch the water. Stir constantly with a silicone spatula until the butter is melted and the sugar has dissolved, about 2 minutes. Remove the bowl from the pan, stir in the chocolate and let stand for 1 to 2 minutes to allow the chocolate to soften.

Using the spatula, stir the mixture until smooth. Add the hot coffee and vanilla; stir until the glaze is smooth and shiny (the mixture may appear broken at first but will eventually come together). The glaze should have the consistency of pourable yogurt. If it is too thick, thin it by stirring in additional hot coffee 1 teaspoon at a time, adding no more than 1 tablespoon. Drizzle the glaze over the top of the cake, allowing it to run down the sides. Let stand for at least 15 minutes before serving.