Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In a separate bowl, whisk the eggs, buttermilk, oil, and vanilla extract until combined.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Slowly stir in the hot coffee until the batter is smooth (it will be thin — this is normal).
Divide the batter evenly between the two prepared pans.
Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool the cakes in their pans for 10 minutes, then transfer to a wire rack to cool completely.
To make the frosting, beat the softened butter until creamy. Add the cocoa powder and mix until combined.
Gradually add the powdered sugar, alternating with the milk, beating well after each addition.
Add the vanilla extract and salt, then beat on high speed for 2–3 minutes until light and fluffy.
Place one cake layer on your serving plate. Spread a layer of frosting on top, then add the second layer.
Apply a thin crumb coat of frosting over the entire cake and refrigerate for 15 minutes.
Apply the remaining frosting to the top and sides for a smooth or rustic finish.