Line a baking sheet with parchment paper.
Break the 9 graham cracker sheets in half to make 18 graham cracker squares. Set 9 squares aside and place the other 9 on the prepared baking sheet.
Add 2 marshmallows on each graham cracker square on the baking sheet.
Set the oven to broil and place the baking sheet in the oven to toast the marshmallows under the broiler for 30 to 90 seconds, until golden brown. Watch the marshmallows very closely and do not walk away from the oven; they will burn very fast, so check them every 10 to 15 seconds.
Remove the baking sheet from the oven as soon as the marshmallows are toasted (do not let them burn). Top each s'more with an unwrapped fun-size Hershey's bar.
Turn the oven off, then place the baking sheet back into the oven, which should still be warm from the broiler, just until the chocolate is melted (about 30 to 60 seconds if the oven is off but warm).
Remove the baking sheet from the oven and place the reserved graham cracker squares on top of the chocolate to form the s'mores.
Serve the s'mores immediately or cool completely before serving.