Set 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.