1Heat oven to 350°F. Spray bottom only of 13x9-inch pan with cooking spray. In large bowl, mix butter, egg and vanilla until blended. Stir in cookie mix until soft dough forms. Press dough evenly in pan.
2Bake 20 to 25 minutes or until top is golden brown and center puffs slightly. Immediately sprinkle chocolate chips over crust. Let stand 3 to 5 minutes or until chocolate begins to melt. Gently spread chocolate evenly over crust.
3Set oven control to broil. Sprinkle nuts and marshmallows over melted chocolate. Broil with top 5 to 6 inches from heat 20 to 30 seconds or until marshmallows are toasted. (Watch closely; marshmallows will brown quickly.) Cool 30 to 45 minutes to serve warm or cool completely, about 2 hours. For bars, cut into 6 rows by 4 rows. Store tightly covered.