Heat oven to 325°F. Line 8-inch square pan with foil, extending foil over sides for easy removal from pan. Bake brownies as directed on box for basic recipe in 8-inch pan. Top hot brownies evenly with peanut butter cup candies, 4 rows by 4 rows; press down slightly. Cool completely.
In large bowl, beat butter and powdered sugar with electric mixer on medium speed until smooth and creamy. Gradually beat in milk. Add peanut butter and vanilla; beat until spreading consistency, adding more milk if necessary. Spread evenly over brownies.
Top with marshmallows, peanuts and baking bits; press down slightly. Refrigerate 30 minutes. Remove from pan by foil edges. For brownies, cut into 6 rows by 6 rows. Cover and refrigerate any remaining brownies.