Preheat oven to 350°F (175°C). Grease two 8-inch round cake pans and dust with cocoa powder. In a large bowl, whisk together flour, sugar, cocoa, baking soda, baking powder, and salt.

In a second bowl, whisk buttermilk, oil, eggs, and vanilla until smooth.

Stir wet mixture into dry ingredients until just combined, then pour in hot coffee and stir until batter is smooth and thin.

Divide batter evenly between pans. Bake 30 to 35 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Cool in pans 10 minutes, then turn out onto a rack to cool completely.

In a saucepan over medium heat, cook butter, cocoa, and milk, stirring constantly, until the mixture boils and is smooth, about 3 minutes. Remove from heat and let cool 5 minutes, then beat in powdered sugar, vanilla, and salt until glossy and spreadable.

Place one cooled cake layer on a plate, spread with one-third of the frosting, top with second layer, and frost top and sides. Serve right away or let set for an hour.