Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides.

Make the crust: In a medium bowl, combine graham cracker crumbs, melted butter, and granulated sugar. Stir until well combined. Press the mixture evenly into the bottom of the prepared baking pan.

Bake the crust for 8-10 minutes, until lightly golden. Remove from oven and let cool slightly.

Make the cookie dough: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. Beat in the egg and vanilla extract until combined.

In a separate small bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

Gently fold in the chocolate chips. Spread about two-thirds of the cookie dough evenly over the baked graham cracker crust.

Sprinkle the mini marshmallows evenly over the cookie dough layer. Dollop the remaining cookie dough over the marshmallows, covering as much as possible.

Bake for 20-25 minutes, or until the edges are golden brown and the top is set. The marshmallows will be puffed and slightly golden.

Let the bars cool completely in the pan on a wire rack before cutting into squares. This allows the layers to set properly.