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

Combine softened butter with brown and granulated sugars in a large mixing bowl and beat until light and fluffy (2-3 minutes).

Add eggs one at a time, mixing well after each addition. Pour in vanilla extract and mix until smooth.

In a separate bowl, whisk together flour, baking soda, and salt. Gradually incorporate into wet ingredients, mixing on low speed until just combined.

Fold in graham cracker crumbs and chocolate chips using a spatula until evenly distributed.

Spread half of the cookie dough evenly into the prepared baking pan and press down gently.

Sprinkle mini marshmallows evenly over the cookie dough base.

Carefully spoon the remaining cookie dough over the marshmallows, optional: add extra chocolate chips and marshmallows.

Bake for 25-30 minutes until golden brown; check with a toothpick for doneness.

Allow bars to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.