Pre-heat oven to 300°F Butter the sides and bottom of a 13x9-inch baking pan.
In a large bowl, combine the flour sugar, cocoa powder, salt, baking powder, and baking soda.
Use an electric mixer to beat the eggs until they become bright yellow and slightly thick. Mix the oil, vanilla, and coffee into the eggs. Mix in the flour, alternately with the buttermilk. Continue to mix until the flour is fully incorporated. Begin with the flour, followed by buttermilk, flour, buttermilk, and finally, flour.
Place the batter in the prepared baking dish. To eliminate air bubbles, I hold the pan a few inches above the counter. Bake for one hour at 300°F. Use a toothpick to test for doneness.