1Heat oven to 350ºF. Stir cookie mix, oil, water and egg in medium bowl with spoon until dough forms. Press in ungreased square pan, 8x8x2 inches.
2Bake 20 to 25 minutes or just until set. Cool completely, about 30 minutes.
3Stir together frosting and marshmallows; spread over bars. Sprinkle with peanuts. For 16 bars, cut into 4 rows by 4 rows. Store tightly covered.