Preheat oven to 350°F and prepare a 13x9 baking dish with non-stick spray. Line baking pan with parchment paper. Sift together the flour, sugar, baking soda and salt into a large mixing bowl and set aside.
Using a medium pot over medium heat, combine the butter, cocoa and coffee, bringing it to a low boil. Whisk until well combined and smooth, then remove from heat.
Slowly pour liquid mixture over the flour ingredients, mixing using a stand mixer or electric hand mixer. Beat in the eggs, buttermilk and vanilla until everything is well incorporated.