Preheat oven to 350 degrees (175 degrees C). Lightly grease two 9 inch layer pans.
Mix flour, baking soda, baking powder, cocoa, and sugar into a large bowl.
Stir in mayonnaise.
Gradually add water and vanilla and blend until smooth.
Pour batter into prepared pans.
Bake at 350 degrees F (175 degrees C) for 30 minutes.
Cool, remove from pans, and frost with your favorite chocolate frosting.
