Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.
Add the eggs one at a time, mixing well after each addition. Then, stir in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the graham cracker crumbs, chocolate chips, and 1 cup of mini marshmallows, ensuring an even distribution.
Spread half of the cookie dough into the prepared baking pan, pressing it down firmly to create an even layer.
Sprinkle the remaining mini marshmallows and chocolate chips over the first layer, then dollop the remaining cookie dough on top. Spread it gently to cover as much of the marshmallows and chocolate as possible.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and the center is set. A toothpick inserted should come out with a few moist crumbs.
Remove from the oven and allow the bars to cool in the pan for at least 15 minutes before using the parchment paper to lift them out. Let them cool completely on a wire rack before cutting into squares.