Preheat your oven to 350°F and line a 9x9-inch baking pan with parchment paper, making sure to leave a bit of overhang on the sides for easy removal later.

In a medium mixing bowl, combine the graham cracker crumbs, flour, baking powder, and salt. Set aside.

In a separate large bowl, beat the butter, brown sugar, and granulated sugar together until the mixture is light and creamy. Add the egg and vanilla and mix until smooth.

Gradually stir the dry ingredients into the wet mixture until a soft dough forms.

Press about two-thirds of the dough evenly into the bottom of your prepared pan. Sprinkle the chocolate chips and mini marshmallows over the top in an even layer.

Break up the remaining dough into small chunks and scatter them over the filling, letting some of the chocolate and marshmallows show through. If using, press a few broken graham cracker pieces on top for extra texture and crunch.

Bake for 22–25 minutes, or until the top is golden and the marshmallows look puffed and toasty. For a picture-perfect finish, gently press Hershey bar pieces into the top while the bars are still warm from the oven.

Let the bars cool completely in the pan. They’ll set up as they cool, making them much easier to slice cleanly into squares.