Preheat broiler. Using a knife, slice each graham cracker in half so that you end up with 8 halves.
Unwrap chocolate bars and slice in half with a knife so that you end up with 4 halves.
Lay out crackers 2 by 2, as if you're making sandwiches onto a baking dish. Place one chocolate half on one side of the graham cracker sandwich. Then place 12 mini marshmallows onto the other half of the graham cracker sandwich. If using large marshmallows, place 1 marshmallow per graham cracker sandiwch.
Broil for 2 - 3 minutes until the marshmallows brown and chocolate melts. Remove from oven. Combine both halves of the graham cracker slices so that you have a s'mores sandwich. Enjoy!
Notes
Have more graham cracker slices on hand, in case some crackers are broken. I had a box where more than half of the crackers were already cracked!