Preheat oven to 350 degrees Fahrenheit. Spray a 9” square baking pan with cooking spray.
Whisk together flour, baking soda, baking powder, salt, sugar and cocoa powder in a medium bowl.
In another bowl, mix together the mayonnaise, water and vanilla extract with a hand mixer.
Gradually add the flour mixture into the wet ingredients, mixing each additional as you add it. Scrape the sides of the bowl as needed. Mix just until blended.