Line a 13x9x2-inch pan with foil, extending the foil over edges of pan; set aside. For crust, in a large bowl stir together 1 1/2 cups powdered sugar, peanut butter, and 6 tablespoons melted butter. Stir in crushed cookies. Press mixture evenly onto the bottom of the prepared pan. Chill while preparing the pudding layer.
For pudding layer, in a small bowl combine milk and pudding mix; set aside. In a large mixing bowl beat 3/4 cup butter with an electric mixer on medium to high speed for 30 seconds. Beat in pudding mixture and vanilla until smooth. Gradually add 3 1/2 cups powdered sugar, beating until smooth. Spread mixture over crust.
In a small microwave-safe bowl microwave butterscotch pieces and shortening on 100 percent power (high) about 1 minute or until melted, stirring every 30 seconds. Pour mixture into a heavy resealable plastic bag; snip a small hole in one corner of the bag. Drizzle mixture in a crisscross pattern over pudding layer.
Cover and chill for at least 2 hours. Using the edges of the foil, lift uncut bars out of pan. Cut into bars or small triangles.