Heat oven to 325°F (350°F for glass pan). Grease bottom only of 8- or 9-inch square pan with shortening or cooking spray. Make brownie batter as directed on box. Stir in 2/3 cup of the chocolate chips. Spread in pan.
Bake brownies as directed on box. Immediately after removing from oven, sprinkle with miniature marshmallows, remaining 1/3 cup chocolate chips, and the peanuts. (For softer marshmallows, cover pan with cookie sheet about 2 minutes.) Cool completely on cooling rack, about 2 hours. Cut into 4 rows by 4 rows with knife dipped in hot water. Store tightly covered
