Preheat oven to 350 and line your baking dish with parchment paper.
In a large bowl, cream the butter and both sugars with a mixer.
Add the egg and vanilla and mix to combine.
In a small bowl, whisk flour, graham cracker crumbs, baking powder and salt.
Add dry mix to wet and mix on low until combined.
Form the dough into a ball with your hands and divide it in half.
Press 1/2 the dough into the bottom of the dish.
Place chocolate bars on top, side by side.
Spread marshmallow over chocolate.
Spread remaining dough on top evenly covering the entire dish.
Bake for 30-35 minutes or until golden brown.
Cool completely - I let mine cool overnight usually.
Cut into 9 or 16 bars, depending upon how big you'd like them.