Cover a baking sheet in foil or parchment paper. Split each of the 9 graham cracker sheets in the sleeve into two squares and lay out all squares onto the sheet. 9 will be used as lids, and 9 as bottoms.
Break Hershey’s bar into bricks of two and lay on each of the 9 squares that will be the bottom of each s'mores sandwich.
Lay marshmallows across each square, both those with chocolate and without. Smush them down or pull apart slightly to make the surface area cover more evenly.
Broil on high for 1-3 minutes, taking care to keep watch to prevent burning.
Remove from the oven and cool for a minute before topping the sandwiches with the graham craker lid and serving warm.