Preheat the oven to 350 degrees F. Center an oven rack in the middle position.
Line the bottom of two 9-inch round cake pans or one 9X13-inch pan (preferably aluminum, not glass) with parchment paper. Lightly grease the bottom and sides with nonstick cooking spray. Set aside.
Sift the flour, sugar, cocoa, baking soda, baking powder, and salt into a large bowl.
Add the water, buttermilk, oil, eggs, and vanilla.
Using a handheld electric mixer or whisk, mix the ingredients until well-combined.
Divide the batter evenly between the prepared pans.
Bake the cakes for 28-35 minutes until the top springs back lightly to the touch and a toothpick inserted into the middle comes out clean or with moist crumbs.
Let the cakes cool for 2-3 minutes in the pan(s). Run a thin knife around the edge and turn onto a cooling rack to cool completely.
Notes
Oil: use a neutral flavored oil for the cake (not an oil with a strong flavor like olive oil)Cocoa Powder: I always use natural, unsweetened cocoa powder (like Hershey's brand) in this cake, but you can sub in Dutch-process cocoa if you like (the cake may not rise quite as high and the texture of the crumb may change a bit). Glass Pan: if baking the cake in a 9X13-inch glass pan, reduce the oven temperature to 325 degrees F and keep an eye on the baking time.