Grease 3 8 inch round cake pans, line with rounds of parchment, grease again and dust with flour. I didn't use the parchment and regretted it! It's a sticky cake.
In a large mixing bowl, combine all dry ingredients (flour through cocoa powder). Stir very well.
In a large measuring cup, combine buttermilk, egg, oil and vanilla. Add liquid ingredients to dry ingredients and stir until blended. Pour in the hot coffee and stir until blended, then beat on low speed with an electric mixer for about 30 seconds or until velvety smooth.
Divide the batter evenly between three pans and bake for 25 to 30 minutes. The original recipe said 35 to 40 but that would have been WAY too long, so start checking at 25..
Allow the cakes to cool for 10 minutes, then loosen from pans and let cool completely.
Prepare the icing. Combine softened butter and 1 cup of the sugar and stir until blended, then stir in cocoa powder and remaining sugar. Add the sour cream and stir into a big goopy blob, then add 2 tablespoon of milk and vanilla and stir well. Continue adding milk and beating with an electric mixer, scraping side of bowl often, until icing is smooth and creamy. Add a pinch of salt (if you used unsalted butter) and beat well. Frost the cake.