Mix the following dry ingredients together: flour, baking soda, salt, and cocoa. If you do not have a sifter, blending this with your mixer will suffice.
Cream the sugar, mayonnaise, water and vanilla together.
Combine dry and creamed mixtures; stir until well blended.
Pour batter into greased and floured 13x9x2 pan. Bake at 350°F. for about 20 to 30 minutes depending on your oven or until a toothpick inserted in the center comes out clean and cake slightly pulls away from the sides of the pan.