Preheat your oven to 350°F (175°C). This ensures your cookie bakes evenly and achieves that perfect golden-brown crust.
In a mixing bowl, combine the melted butter, brown sugar, granulated sugar, and vanilla extract. Whisk until the mixture is smooth and well-combined.
Add the egg yolk (or whisked whole egg) to the butter and sugar mixture. Whisk again until fully incorporated.
In a separate bowl, mix the flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, stirring until just combined. Be careful not to overmix!
Gently fold in the semi-sweet chocolate chips, ensuring they’re evenly distributed throughout the dough.
On a parchment-lined baking sheet, scoop half of the cookie dough onto one side. Flatten it slightly to create a base. Break the graham cracker in half and place it on top, followed by the Hershey's chocolate segments and marshmallows. Finally, scoop the remaining cookie dough on top to cover the filling.
Bake in the preheated oven for 10-12 minutes, or until the edges are golden brown and the center looks slightly underbaked. This will ensure a chewy texture.
Allow the cookie to cool on the baking sheet for a few minutes before transferring it to a wire rack. Enjoy warm for the best gooey experience!