Preheat oven to 350 degrees. In an 8-inch square baking pan, whisk together flour, sugar, cocoa powder, baking soda, and salt.
Make a well in center of flour mixture and add oil, vanilla, vinegar, and water. Whisk until well combined. Bake until a toothpick inserted in center comes out clean, 35 to 40 minutes. Let cool completely in pan on a wire rack.
