Preheat oven to 350°F. Grease 2 9-inch round cake pans and line with circles of wax paper or parchment. Grease again and dust with flour.
In a large mixing bowl bowl, stir together flour, sugar, cocoa, baking powder, baking soda and salt.
Make a well in the center and add eggs, oil, buttermilk and vanilla. With a heavy duty scraper, stir mixture, scraping up dry mixture from the bottom to ensure that it is incorporated. Add the hot coffee and stir until thick and well blended. With an electric mixer, beat on medium speed for about 30 seconds or until well blended.
Divide evenly between 2 cake pans and bake on center rack for 35 minutes or until cakes spring back when touched. Allow the cakes to cool on a rack for about 15 minutes, then carefully invert and let cool completely.
To make the frosting, combine the softened butter, half of the sugar and the cocoa powder in a large mixing bowl. Stir until mixed, it will be thick and pasty. Stir in the vanilla, then continue adding the sugar, stirring until it’s very thick. Add the buttermilk and stir, then continue adding milk and stirring, scraping bowl often, until it’s slightly creamy. Now begin beating, adding more milk as necessary, until it is smooth and creamy.
Level the cake if you need to (though this one bakes up fairly evenly), then spread cakes with frosting and stack.