Preheat the oven to 325 degrees F. Butter 2 (9-inch) cake pans and line the bottoms with parchment paper.
Whisk together the flour, baking powder, baking soda and salt in a medium bowl.
Melt the 12 tablespoons of butter in a medium saucepan over medium heat. Whisk in the cocoa powder and cook for 1 minute. Remove from the heat, add the sugars and whisk until the sugar is dissolved. Add the coffee, buttermilk, eggs and vanilla extract and continue whisking until smooth and just combined.
Divide the batter evenly between the 2 pans and bake on the middle rack until a toothpick inserted into the center comes out with a few moist crumbs attached, about 42 minutes. Let cool in the pans on a baking rack for 20 minutes. Then invert the cakes onto the baking rack and let cool for at least 1 hour before frosting.