1. Preheat the oven to 350°F. Grease two 9-inch round cake pans, line the bottoms with parchment paper, and lightly grease the parchment.

2. In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.

3. In a separate bowl, whisk the eggs, buttermilk, vegetable oil, and vanilla extract until smooth.

4. Add the wet ingredients to the dry ingredients and mix until just combined.

5. Slowly pour in the hot coffee or hot water and stir until the batter is smooth. The batter will be thin.

6. Divide the batter evenly between the prepared cake pans.

7. Bake for 28 to 34 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

8. Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack and cool completely.

9. To make the frosting, beat the softened butter until creamy.

10. Add the powdered sugar, cocoa powder, melted dark chocolate, vanilla extract, salt, and 3 tablespoons of heavy cream. Beat until smooth and fluffy. Add more cream as needed for a spreadable texture.

11. If needed, level the cake layers with a serrated knife.

12. Place one cake layer on a serving plate and spread frosting over the top.

13. Add the second cake layer and frost the top and sides of the cake.

14. Let the cake sit for 15 to 20 minutes before slicing for the cleanest finish.