In a mixing bowl, combine flour, sugar cocoa and salt. Add oil, eggs and vanilla; beat at medium speed for 3 minutes. Stir in walnuts if desired. Pour mixture into a greased 13x9x2" pan. Bake for 30 minutes or until brownies test done. Cool completely.
In a medium bowl, cream together the butter and sugars. Add milk and vanilla. Beat in flour. Spread over cooled brownies and chill until firm.
For glaze, melt chocolate chips and shortening in a saucepan until smooth. Spread over filling. If desired, sprinkle with walnuts while the chocolate is still soft. Allow chocolate to cool before cutting brownies.