Preheat the oven and line the pan

• Preheat the oven to 175 °C.
• Line an 8x8-inch baking pan with parchment paper, leaving some overhang for easy removal.

Whisk the wet ingredients

• In a large bowl, whisk together the melted butter, brown sugar and granulated sugar until smooth.
• Add the egg, egg yolk and vanilla extract and whisk until fully combined.

Make the cookie dough

• Stir in the flour, baking soda and salt just until no dry streaks remain; do not overmix.
• Fold in most of the chocolate chips, reserving some for the filling and topping.

Layer the bars

• Line the bottom of the pan with graham crackers.
• Gently spread the cookie dough evenly over the graham crackers, pressing into an even layer.
• Add the marshmallows and some chocolate chips, pressing down gently.
• Dollop the remaining cookie dough over the marshmallows, leaving some marshmallows exposed, and sprinkle extra chocolate chips on top.

Bake

• Bake until the edges are golden brown and the center is just set.

Add the chocolate topping and cool

• Let the bars cool in the pan for 15-20 minutes.
• While still warm, press pieces of the Hershey's chocolate bar into the top.
• Remove from the pan, transfer to a wire rack and let cool completely before cutting.