Combine cocoa, sugar, salt, flour, and soda (I just use my Kitchen-Aid mixer). Add the oil, buttermilk, and eggs. Mix well, then add the water. Put in a greased and floured bundt pan.
Bake at 350 degrees for about 40 minutes. Remove cake from pan while warm, not hot.
FOR ICING: Boil the sugar, cocoa, milk, and margarine in a saucepan on the stove for 1 to 2 minutes. Add the vanilla. Stir until cool or thick enough to stick on cake. Put on quickly or it will get too thick.