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-in.x10-in.x1-in. baking pan. Bake at 325 degrees for 40-45 minutes or until brownies test done. Cool for 10 minutes. Meanwhile, for icing, melt the butter and chocolate. Place in a mixing bowl. Add half the confectioners' sugar; mix well. Add milk, vanilla and remaining sugar and beat until smooth. Spread immediately over warm brownies. Sprinkle with nuts if desired.