Preheat oven to 375F, grease and flour 13x9 pan. Boil 1 C margarine, 1 C water, and 4 Tbsp cocoa. Mix 2 C sugar, 2 C flour, 1/2 tsp salt, and 1 Tbsp baking soda in large bowl. Mix 2 eggs and 1/2 C sour cream in small bowl. Add cooled cocoa mix to egg mixture slowly to temper the eggs. Add wet mix to dry and beat well. Pour into pan and bake about 30 minutes till cake springs back when touched. Frosting: boil 1/2 C margarine with 4 Tbsp cocoa, add 6 Tbsp milk, 3 1/2 C powdered sugar, and 1/2 tsp vanilla. Mix and pour over cake while warm.