Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, beat together the softened unsalted butter, light brown sugar, and sugar using an electric mixer until the mixture is light and fluffy.
Add the eggs, one at a time, mixing well after each addition. Then, stir in the vanilla extract until fully combined.
In a separate bowl, whisk together the all-purpose flour, graham cracker crumbs, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, mixing just until combined.
Gently fold in the chocolate chips, mini marshmallows, and chopped chocolate bar until evenly distributed throughout the dough.
Spread half of the cookie dough into the prepared baking pan, pressing it down evenly. Sprinkle the broken graham cracker pieces on top.
Layer the mini marshmallows over the first layer of dough, then crumble the remaining cookie dough on top, covering the marshmallows as best as you can.
Bake in the preheated oven for 25-30 minutes or until the edges are golden brown and the center is set. The marshmallows will puff up and create a beautiful topping.
Allow the bars to cool in the pan for at least 15 minutes before using the parchment paper to lift them out. Slice into squares and enjoy warm or at room temperature!
