LINE 13x9 pan with parchment paper sprayed with cooking spray. Place 13-1/2 (or however many to cover bottom)squares of graham crackers in pan, with sides overlapping slightly. Break remaining crackers into pieces for top.
MICROWAVE butter and chocolate squares in microwaveable bowl on high for about 2 minutes. Stir until chocolate is completely melted.
COMBINE sugar, eggs and vanilla into a bowl; mix well. Add chocolate mixture. Stir in flour. Pour over graham crackers in pan. Sprinkle chocolate chips on top of batter.
BAKE 30-32 minutes. Remove from oven and sprinkle marshmallows then broken pieces of Hersheys bars. Bake 3 to 5 more minutes or until marshmallows begin to puff. Press reserved graham crackers into marshmallows. Cool slightly. Use edges of parchment paper to lift brownies from pan before cutting to serve.
To cut, dip a knife in hot water, dry it off, and cut. OR use a plastic knife to cut (heard that tip from somewhere). I actually just tried it, and it does work well. But the brownies were from last night and they are no longer warm. Give it a try and let me know!!