Preheat the oven to 350°F (175°C) and grease two 9-inch round cake pans or one 9×13 pan. Line bottoms with parchment.

In a large bowl, whisk flour, cocoa powder, sugar, baking powder, baking soda, and salt until evenly combined.

Add eggs, vegetable oil, buttermilk, and vanilla. Stir gently until mostly smooth, with some streaks remaining.

Pour in hot coffee and stir again. Batter will be thin—that's perfect.

Divide evenly into pans. Bake round cakes for 25–30 minutes or 9×13 for 30–35 minutes, until a toothpick comes out with a few moist crumbs.

Cool cakes for 10 minutes in pans. Invert onto wire racks and cool completely.

Make the Ganache: Place chocolate in a heat-safe bowl. Heat cream just to a simmer, then pour over chocolate. Let sit 2 minutes. Stir from center until smooth and shiny. Stir in butter if using.

Let ganache cool for 20–30 minutes until spreadable.

Assemble the Cake: Level cakes if desired. Place the first layer on a serving plate and spread a thin ganache layer. Add the second layer and pour the rest of the ganache on top. Let it drip naturally, then smooth as desired. Chill briefly if needed to set.