Preheat your oven to 350°F. Grease two 9-inch round cake pans and line the bottoms with parchment paper.
In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, and salt until evenly combined.
In another bowl, beat the eggs, buttermilk, vegetable oil, and vanilla extract until smooth. Gradually add this mixture to the dry ingredients and stir until combined.
Slowly pour in the hot coffee while mixing. The batter will become quite thin, which is normal. Continue mixing just until everything is incorporated.
Divide the batter evenly between the prepared pans. Bake for approximately 30–35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to rest in their pans for several minutes, then transfer them to a wire rack to cool completely before frosting.
Beat the softened butter until creamy. Gradually add the confectioners' sugar and cocoa powder. Pour in the heavy cream and vanilla extract, then continue beating until the frosting becomes smooth, light, and fluffy.
Place one cooled cake layer on a serving plate and spread frosting evenly over the top. Add the second cake layer and cover the top and sides with the remaining frosting. Slice and serve.
