Prepare brownie mix according to package instructions; spread in a greased 13"x9" baking pan.
Sprinkle one cup mini marshmallows, one cup chocolate chips and 1/2 cup peanuts over batter.
Bake at 350 degrees for 28 to 30 minutes. Remove from oven; sprinkle with remaining marshmallows, peanuts and chocolate chips. Let cool completely before cutting into squares.