Heat oven to 350°F. Line 9-inch square pan with foil; spray foil with cooking spray. Make brownies as directed on box, using water, oil and eggs. Pour batter into pan.
Bake 34 to 37 minutes or until almost set in center. Immediately sprinkle brownies with marshmallows, peanut butter chips, peanuts and cookies. Bake 2 to 3 minutes longer or until marshmallows puff.
In small microwavable bowl, mix frosting and peanut butter. Microwave uncovered on High 30 seconds or until pourable. Drizzle over brownies. Cool completely on cooling rack. Cut into 4 rows by 4 rows.