Preheat the oven to 350°F (175°C).
Grease two 9-inch round cake pans and line the bottoms with parchment paper.
In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Add eggs, buttermilk, vegetable oil, and vanilla extract.
Mix until the ingredients are combined.
Slowly pour in the hot coffee and hot water while mixing on low speed.
Continue mixing just until the batter is smooth. The batter will be thin.
Divide the batter evenly between the prepared cake pans.
Bake for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the cakes cool in the pans for about 10 minutes.
Carefully transfer the cake layers to a wire rack and allow them to cool completely.
For the frosting, beat softened butter until smooth and creamy.
Gradually add powdered sugar and cocoa powder, mixing on low speed.
Add heavy cream, vanilla extract, and salt.
Beat until smooth and creamy.
Add the melted, slightly cooled chocolate.
Beat for another 2–3 minutes, until fluffy and spreadable.
Place one cooled cake layer on a serving plate.
Spread a generous layer of chocolate frosting over the top.
Place the second cake layer on top.
Cover the top and sides of the cake with the remaining chocolate frosting.
Decorate with chocolate shavings if desired.
Slice and serve once the frosting has set.