In a mixing bowl, cream butter and sugar. Add eggs. Combine flour, cocoa and salt; add to creamed mixture and mix well. Stir in vanilla and walnuts. Spread into a greased 15″ x 10″ x 1″ baking pan. Bake at 325* for 40-45 minutes or until brownies test done. Cool for 10 minutes. Meanwhile, for icing, melt the butter and the chocolate. Place in a mixing bowl. Add half of the powdered sugar, mix well. Add milk vanilla and remaining sugar and beat until smooth. Spread immediately over warm brownies. Sprinkle with nuts, if desired.