For the cake: Combine sugar, flour, cocoa, baking soda and salt in a large mixing bowl. Mix well. Heat margarine and water in a saucepan until boiling. Pour over dry ingredients and mix. Beat in buttermilk, eggs, and vanilla. Pour thin batter into greased and floured 15 x 10 inch pan. Bake at 400 deg f for 20 minutes. Serv this cake warm or cold. For the icing: Combine margarine, cocoa, and milk. Heat to boiling. Pour over confectioners sugar and nuts. Mix well. Spread over hot cake. Makes 18 servings.