Heat oven to 350°F (325°F for dark or nonstick pan). Make brownies as directed on box for 9-inch square pan. After removing pan from oven, set oven control to broil.
Immediately sprinkle marshmallows and snack mix over warm brownies. Broil with top 4 to 5 inches from heat 30 to 60 seconds or until marshmallows are golden brown. (Watch carefully; marshmallows and snack mix will brown quickly.) Sprinkle with candy. Cool about 30 minutes. Cut into 4 rows by 4 rows. Store loosely covered.