Preheat oven to 350 °F. Lightly grease an 8-inch baking pan
In a large mixing bowl, beat butter and sugar at medium speed, until creamy.
Add egg and vanilla, beat until combined.
In a small bowl, combine flour, graham cracker crumbs, baking powder, and salt. Add to butter mixture, and beat until combined.
Press half of dough into the pan, spreading evenly.
Lay chocolate bar on top of dough, breaking apart as needed to create an even layer of chocolate.
Spread marshmallow creme over chocolate bars.
Spread remaining dough over marshmallow creme and chocolate bars.
Bake 30 to 35 minutes until lightly golden on top. Do not overtake.
Allow to cool before slicing into squares