Sift dry ingredients together. Mix mayo and sugar. Alternate mixing mayo mix, dry ingredients and water til blended well. Then add vanilla and mix well. Pour into 9x13 greased sheet pan. Bake until toothpick comes clean. Bake @ 350 for 15-20 min or when the toothpick comes out clean. Determine by your oven.
While cooling mix your topping. In small boiler add sugar, evap. milk, cocoa and butter. Boil, stirring costantly, for 3 min. Then add vanilla and pinch of salt--mix well. Add nuts if you choose.
With butter knife make poke holes in cake to absorb topping. Pour topping into center of cake spreading all over and over the edges. Let sit till cools. Eat!