Line a 9x13 baking dish with parchment paper or aluminum foil
In a large bowl, cream together butter and sugar until light.
Add eggs and vanilla and beat.
In a large bowl, whisk together flour, graham cracker crumbs, baking powder, and salt.
Add dry ingredients to butter mixture and mix on a low speed until combined.
Divide dough in half and press half of the dough into an even layer on the bottom of the prepared baking pan.
Place Reese's peanut butter cups in rows on the dough then fill in the gaps with Hershey chocolate bar pieces.
Spread marshmallow fluff on top of the chocolate, making sure to cover all of it.
Place remaining dough on top of the marshmallow in a single layer.
Bake for 30 to 30 minutes or until lightly browned.
Allow bars to cool completely then cut into bars.
Line the baking dish with parchment paper, with a bit of extra on the ends,so that when your bars are done, you can simply pull the parchment paper out and cut out of the dish.Flatten the remaining doughbefore you put it on top then just patch it together to cover most of the marshmallow. It doesn’t need to be totally covered.Cool completely before cutting into the bars. I know you’ll want to eat them right away but they need to cool or you’ll burn your little tongue on the marshmallow and they may fall apart.