Preheat the oven to 350 F. Grease a 10 to 12-cup Bundt pan with butter and coat with cocoa powder.
Whisk the dry ingredients including flour, cocoa, sugars, baking powder, baking soda, and salt in a large bowl.
Combine the wet ingredients by whisking buttermilk, oil, eggs, and vanilla in a separate bowl.
Make the batter by stirring wet ingredients into dry ingredients just until combined, then slowly whisking in the hot coffee.
Bake for 38 to 45 minutes, until a skewer inserted into the center comes out with a few moist crumbs.
Cool the cake in the pan for 15 minutes, then invert onto a wire rack and cool completely.
Glaze and serve by whisking powdered sugar, milk, and vanilla until smooth, then spooning it over the cooled cake.