In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, cocoa and salt; gradually add to creamed mixture and mix well. Stir in walnuts.
Spread into a greased 15x10x1-in. baking pan. Bake at 325° for 40-45 minutes or until top springs back when lightly touched.
Meanwhile, melt butter in a saucepan over low heat. Remove from the heat; stir in milk and cocoa until smooth. Return to the heat and bring to a boil; boil for 4 minutes, stirring constantly. Remove from the heat; stir in confectioners' sugar and vanilla until smooth. Frost warm brownies. Sprinkle with walnuts if desired. Cool on a wire rack before cutting.