Preheat oven to 300 degrees. Grease the bottom and sides of a 13x9 inch baking pan.
Mix the flour sugar, cocoa, salt, baking powder and baking soda together in a large bowl.
Beat the eggs with an electric mixer until they are bright yellow and slightly thick. Add the oil, coffee and vanilla to the eggs, mix until well incorporated. Add the flour to the mix, alternating with the buttermilk, mixing after each addition until just incorporated. Start with the flour first, then buttermilk, flour, buttermilk and end with flour.
Pour the batter into the prepared baking pan. I always hold the pan up a few inches from the counter and let it fall to get rid of air bubbles in the batter. Bake at 300 degrees for 1 hour. Test for doneness with a toothpick.