Preheat oven to 180ºC . Butter a 9 inchx5 inch loaf pan. Line the bottom with wax paper and butter the wax paper. You may also use two 20 cm round cake pans, line with butter wax paper.
In a large bowl, combine the flour, sugar, salt, baking soda, and cocoa and mix well. [Or, whiz in food processor.] Add the water and vanilla and beat until smooth. [Add and whiz.] Add the mayonnaise and beat until the batter is complete smooth and free of lum ps. [Add and whiz.] Transfer the batter to the pan and bake for 50 min for the loaf, 30-35min for t he layers, or until a cake tester comes out dry.
Cool for at least 20 min on a rack before removing from the pan. Remove from the pan and peel off the wax paper. The cake has such a rich fudgy taste that a little confectioners' sugar sprinkl ed over the top to give it a finished look is all that it needs.