Preheat oven to 375 degrees. Coat a 9×13 baking pan with cooking spray.
Set aside.
Sift or whisk together flour, baking soda, and cocoa powder. Set aside.
In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat together water, sugar, mayonnaise, eggs (optional) and vanilla.
Slowly add dry ingredients, mixing well after each addition.
Mix dry ingredients and wet ingredients in separate bowls. Slowly add the dry ingredients to the wet, mixing while you add them.
Pour into prepared 9×13 baking dish.
Bake for 30 minutes or until it passes the toothpick test. Allow cooling completely.
