Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the melted and slightly cooled butter, brown sugar, granulated sugar, and vanilla extract. Whisk them together until well blended.
Add in the egg yolk (or whisked whole egg) to the mixture and whisk until fully incorporated.
In another bowl, whisk together the all-purpose flour, baking soda, and salt. Slowly add the dry mixture to the wet ingredients, stirring until just combined.
Gently fold in the semi-sweet chocolate chips and crushed graham cracker until they're evenly distributed throughout the dough.
Scoop the dough onto a parchment-lined baking sheet, forming a single cookie that’s about 1/2 inch thick.
Top the cookie with segments of the Hershey's bar and place the mini marshmallows on top. Press them down slightly to ensure they stick.
Place the baking sheet in the preheated oven and bake for 10-12 minutes, or until the edges are golden but the center remains soft.
For that perfect toasted marshmallow finish, switch the oven to broil for an additional 1-2 minutes. Keep a close eye to prevent burning!
Remove the cookie from the oven and let it cool for a few minutes before enjoying.
