Preheat oven to 350°F.
CRUST: Beat butter, peanut butter and sugar in bowl until creamy. Stir in flour, corn starch and salt until well combined and crumbly. Press evenly into bottom of greased 8-inch square baking dish. Bake for 12 to 15 minutes or until pale golden around edges; set aside.
TOPPING: Stir together corn syrup, corn starch, baking powder and salt in a small bowl until smooth. Stir in butter, eggs and vanilla until well blended. Pour over crust. Sprinkle peanuts, peanut butter chips and chocolate chips over top.
Bake for 20 to 25 minutes or until golden and just set in the center. Let cool completely in pan on rack. Cut into squares