1. Heat oven to 350 degree F. Arrange graham crackers evenly in 15 x10x1-inch jelly-roll pan. Sprinkle marshmallows, then chocolate chips on top.
2. Combine butter and brown sugar in medium microwave proof bowl. Microwave 1-1/2 to 2 minutes on 100 percent power (High) until butter is melted. Stir to blend. Spoon evenly over marshmallows and chips. Bake 8 to 10 minutes or until barely golden. Cool completely in pan on wire rack. Cut into squares. Makes 24 bars.