Preheat your oven to 350°F (175°C). Grease an 8×8-inch square baking pan or line it with parchment paper, leaving an overhang on the sides for easy removal of the bars later.

In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together until the mixture is light and fluffy. This should take about 2 to 3 minutes using an electric mixer.

Add the egg and vanilla extract to the butter mixture. Beat again until everything is well combined and smooth, scraping down the sides of the bowl as needed.

Whisk together the flour, graham cracker crumbs, baking powder, and salt in a separate small bowl. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a thick dough forms.

Press about two-thirds of the cookie dough into the bottom of your prepared baking pan. Spread it out evenly to create a solid base for your toppings.

Sprinkle the chocolate chips evenly over the dough base. Top the chocolate with a generous layer of mini marshmallows, pressing them down slightly so they stay in place.

Take the remaining cookie dough and flatten small pieces of it with your hands. Place these “patches” of dough over the marshmallow layer. It is okay if some marshmallows are still showing. Bake for 20 to 25 minutes until the top is golden brown and the marshmallows are toasted.

Remove the pan from the oven and place it on a wire rack. Let the bars cool completely before cutting them into squares. This allows the marshmallow and chocolate layers to set so they do not ooze out too much.