Preheat your oven to 350°F (175°C) and prepare a 9x13 inch baking pan by greasing it or lining it with parchment paper.

In a large mixing bowl, cream together softened unsalted butter, brown sugar, and granulated sugar until light and fluffy (about 3-4 minutes).

Add in the large eggs one at a time, mixing well after each addition. Stir in the vanilla extract until evenly incorporated (1-2 minutes).

In a separate bowl, whisk together all-purpose flour, baking soda, and salt. Gradually add this to the wet ingredients, stirring gently until combined.

Fold in the graham cracker crumbs and chocolate chips until evenly distributed throughout the dough.

Spread half of the cookie dough mixture into the prepared baking pan and layer mini marshmallows over it.

Dollop the remaining cookie dough over the marshmallows and sprinkle with extra chocolate chips and marshmallows on top.

Bake in the preheated oven for 25-30 minutes until edges are golden brown and a toothpick comes out clean.

Let the bars cool for about 10 minutes at room temperature, then transfer to a wire rack to cool completely before cutting into squares.