Preheat your oven to 350°F (175°C) and grease your cake pan(s).
In a large bowl, whisk together sugar, flour, cocoa powder, baking powder, baking soda, and salt.
Add eggs, buttermilk, oil, and vanilla extract. Mix until well combined.
Stir in boiling water (or hot coffee) until the batter is smooth and thin.
Pour the batter into the prepared pan(s) and bake for 30-35 minutes.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack.
For the frosting, mix melted butter, cocoa powder, powdered sugar, milk, and vanilla until smooth.
Spread frosting over the cooled cake, slice, and enjoy!