In the bowl of an electric mixer, beat the sugar and mayonnaise until blended. Add the vanilla and blend. Sift together the flour, cocoa, baking soda and salt and add it to the mayonnaise mixture in 3 batches alternating with the buttermilk.
Pour into a greased and floured 9 by 9 by 2-inch baking pan. Bake for 45 to 50 minutes or until a cake tester comes out clean.
Let cool on a wire rack, dust with confectioners' sugar and garnish with whipped cream before serving.