Preheat oven to 350°F. Spray a 9x9-inch baking dish with nonstick cooking spray.

In a medium mixing bowl, whisk together the flour, sugar, cocoa powder, and baking soda.

Add the water, mayonnaise, and vanilla. Mix until evenly combined.

Pour the batter into the prepared baking pan. Bake for 25-30 minutes, or until a toothpick comes out mostly clean.

Let the cake cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely.