Preheat oven to 350°F (see note). For round pans, spray pans with nonstick cooking spray, then line bottoms with parchment paper. Spray 9 x 13-inch pan with nonstick cooking spray.
Whisk together flour, sugar, cocoa, baking soda, and salt in a large mixing bowl.
Combine water, oil, vinegar, vanilla, and espresso powder (if using) in a large measuring cup.
Add to the dry ingredients and whisk just until combined -- a few lumps are okay.
Divide batter between prepared round pans, or pour batter into 9 x13 pan.
Bake until a toothpick inserted in the center comes out clean, 28 minutes for round pans, or 32-35 minutes for 9 x 13 pan.
Cool on wire rack. When completely cooled, frost.
For layer cake: Cool cakes on a wire rack for 15 minutes, then invert onto the rack, removing pans. Leave cakes upside down (this flattens domed cakes) to cool completely.
It’s really important to get the cake into a preheated oven as quickly as possible once the dry ingredients are mixed with the wet ingredients. You’ll see the batter begin to bubble immediately as the vinegar and baking soda react. This is what leavens the cake.