Preheat oven to 350°F, grease and flour two 8-inch cake pans.
Sift cocoa, flour, baking soda, salt and sugar into a medium bowl, mix well. Combine mayonnaise, vanilla and water in an electric mixer at low speed. Once smooth, slowly add in dry mixture until just incorporated (do not over beat). Pour batter into cake pans and bake for 30 minutes or until done. Cool on wire racks and frost with your favorite chocolate frosting.
In the bowl of an electric mixer, beat butter and cocoa with whisk attachment until smooth. Add vanilla. Add confectioners' sugar alternately with milk until icing reaches a spreadable consistency. (you may not need all 6 cups of sugar; you may need more than 6 tablespoons of milk)