Sift together the first 5 dry ingredients. Add the vanilla, mayo and water to dry mixture - mix well.

Pour into a 9x13 inch greased pan. Bake at 350 degrees for 30 minute.

In a saucepan bring the butter, cocoa and milk to a boil and continue to boil for 3 minutes - stirring constantly. Pour over the powdered sugar and stir well. Let cool and then frost cool cake.