In a mixing bowl, combine sugar, flour, cocoa, and salt. Add oil, eggs, and vanilla; beat in medium speed for 3 minitues. Stir in walnuts, if desired. Pour into a greased 13x9 inch baking pan. Bake at 350 degrees for 30 minutes or until brownies test done. Cool completely. For filling, cream butter and sugars in a mixing bowl. Add milk and vanilla; mix well. Beat in flour Spread over the brownies; chill until firm. For glaze, melt chocolate chips and shortening in a saucepan, stirring until smooth. Spread over filling. Immediately sprinkle with nuts, pressind down slightly/.