Mix together flour, sugar eggs, sour cream, salt and baking soda. In a saucepan, combine butter, cocoa and water. Bring to a boil. Remove from heat. Add to flour mixture and beat well. (You do not have to cool before adding to the flour mixture.) Pour into greased cookie sheet or jelly roll pan. Bake 20 minutes at 350 degrees. For topping, mix powdered sugar, nuts and vanilla. In a saucepan, bring cocoa, butter and milk to a boil. Pour into sugar mixture and mix well. Pour over cake while cake is warm.