Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking pan and line the bottom with parchment paper.

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

In another bowl, combine the mayonnaise, water, and vanilla extract.

Pour the wet mixture into the dry ingredients and stir until just combined.

Pour the batter into the prepared pan and smooth the top with a spatula.

Bake for 30 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes, then turn it out onto a cooling rack to cool completely.

Frost with your favorite icing or enjoy it plain after it has cooled.