Preheat oven to 375 degrees F (190 degrees C). Grease a 15x10 inch jelly roll pan.
Bring 1 cup butter, water and 4 tablespoons cocoa to a boil in a large saucepan. While still hot, remove from heat and add 2 cups flour, 2 cups white sugar, and salt. Mix well. Beat in eggs, sour cream, and baking soda. Do not beat too long.
Pour batter into greased 15x10 inch jelly roll pan. Bake at 375 degrees F (190 degrees C) for 20 to 25 minutes, or until a toothpick inserted into center of cake comes out clean.
To Make Icing: Bring 1/2 cup butter or margarine, 4 tablespoons cocoa and 5 tablespoons milk to a boil. Remove from heat. While still hot, add confectioners' sugar vanilla and nuts. Beat well. Ice cake while icing is still hot.