Preheat oven to 350 degrees. Line a 8-inch square baking pan with foil and grease lightly.
In a large bowl, cream together butter and sugar until light. Beat in egg and vanilla.
In a small bowl, whisk together flour, graham cracker crumbs, baking powder and salt.
Add to butter mixture and mix at a low speed until combined. Divide dough in half and press half of dough into an even layer on the bottom of the prepared pan.
Place chocolate bars over dough. You may not need all of the chocolate, you only want a single layer, so break as needed to fit the pan.
Spread chocolate with marshmallow creme.
Place remaining dough in a single layer on top of the marshmallow layer (most easily achieved by flattening the dough into small pieces and laying them together).
Bake for 30-35 minutes, until lightly browned.
Cool completely before cutting into bars. Makes 24 bars.
Video
Notes
The brand of chocolate bars you buy will determine how many you need, buy a 6 pack to make sure you have enough.
Use binder clips or clothespins to secure parchment paper to baking dish while adding ingredients. Be sure to remove them before baking.
Store cookie bars in airtight container at room temperature or refrigerator.