Preheat oven to 350 degrees.
2 C. flour
1 tsp. baking powder
1 tsp. baking soda
1/2 tsp. salt
1/4 c. cocoa powder
3/4 c. mayonnaise
1 c. sugar
1 c. water
1 tsp. vanilla
Mix dry ingredients in one bowl, set aside. Combine mayo, sugar, vanilla and water. Combine wet and dry ingredients, beat until smooth. Pour into 13x9x2" pan. Bake for 35 minutes or until done. Top should spring back when lightly touched. Do not overbake.