1Heat oven to 350°F. Line 13x9-inch pan with foil, extending foil over sides for easy removal from pan. Grease bottom only of foil with shortening or cooking spray. Make brownies as directed on box. Top hot brownies evenly with peanut butter cup candies, 6 rows by 4 rows; press down slightly. Cool completely.
2In large bowl, beat butter, powdered sugar and milk with electric mixer on medium speed until smooth and creamy. Add peanut butter and vanilla; beat until spreading consistency, adding more milk if necessary. Spread evenly over brownies.
3Top with marshmallows, peanuts and chocolate candies; press down slightly. Refrigerate 30 minutes. Remove from pan by foil edges. For brownies, cut into 8 rows by 6 rows. Cover and refrigerate any remaining brownies.