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

In a large mixing bowl, cream the unsalted butter, brown sugar, and granulated sugar until light and fluffy (about 2-3 minutes).

Add eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually mix this into the butter mixture until just combined.

Fold in the graham cracker crumbs and half of the chocolate chips until evenly distributed.

Press half of the cookie dough into the prepared baking pan evenly. Sprinkle the mini marshmallows over the base layer, then cover with the remaining cookie dough.

Before baking, sprinkle additional chocolate chips and mini marshmallows on top.

Bake for 25-30 minutes until edges are golden brown. A toothpick inserted should come out clean or with moist crumbs.

Remove from the oven and let cool in the pan for about 10 minutes. Use the parchment overhang to lift out and let cool completely on a wire rack before slicing into squares.