In a large bowl, beat the eggs, oil, sugar,and vanilla until well blended. Combine the flour, cocoa, and salt; gradually beat into egg mixture. Stir in walnuts if desired.
Pour into a greased 13-in. x 9-in. baking pan. Bake at 350° for 30 minutes or until brownies test done. Cool completely.
For filling, in a large bowl, cream butter and sugars until light and fluffy. Beat in milk and vanilla. Gradually beat in flour. Spread over the brownies; chill until firm.
For glaze, in a microwave, melt chocolate chips and shortening; stir until smooth. Spread over filling. Immediately sprinkle with nuts, pressing down slightly. Let stand until set.