Preheat oven to 350 F. Grease a 9x13-inch metal baking pan. Set aside.
In a large bowl, whisk together buttermilk and vegetable oil.
Whisk in eggs one at a time.
Stir in the flour, sugar, cocoa powder, baking soda, baking powder, and salt until just combined.
Add hot water and vanilla.
Whisk until just combined. The batter will be thin.
Pour batter into prepared baking dish.
Bake 30-40 minutes, or until wooden toothpick inserted in the center comes out clean.
Cool to room temperature.
Notes
All-purpose flour: I highly recommend weighing your ingredients. To get the weights, click on "metric" immediately above the ingredients. You can also sift or stir the flour to break it up, lightly spoon it into the measuring cup, and level.
Cocoa powder: I use regular unsweetened cocoa powder in this recipe. I’ve also used Hershey’s special dark cocoa powder. However, I haven’t tried Dutch-processed cocoa powder with this cake.
Salt: If you don’t have kosher salt, use 1/2 teaspoon table salt instead.
Buttermilk: Regular or low-fat will work. If you don’t have buttermilk, you can make a quick substitute with this recipe.