Add the melted butter, brown sugar, and granulated sugar to a mixing bowl and mix well.
Beat in the egg and vanilla until well combined.
Stir the flour, graham cracker crumbs, baking soda, and salt into the butter mixture until well combined.
Stir the broken graham cracker into the cookie dough and scoop into 12 evenly sized cookie dough balls.
Place cookie dough on a large baking sheet and bake for 10 minutes.
Remove the cookies from the oven and carefully press a Hershey's bar lightly onto each cookie.
Top each cookie with a marshmallow and return to the oven under the broiler. Broil on high for 1-2 minutes or until marshmallows are toasted to your liking.
Serve immediately.
Video
Notes
To store the cookies, keep in a tightly closed cookie jar. Reheat in the microwave for 10 seconds before serving.