Preheat oven to 350°Grease an 8-inch square baking pan.

In a large bowl, cream together butter and sugar until light. Beat in egg and vanilla. Add flour, graham crackers crumbs, baking powder, and salt, and mix 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. The 2 bars should fit side by side, but if not; break to fit in one layer. Spread marshmallow fluff over chocolate bars.

Place remaining dough in a single layer on top of the fluff. Flatten pieces of dough between your hands and make little discs. Lay these discs over the marshmallow fluff, trying to cover the surface evenly. It's okay if a little bit of marshmallow is peeking through. You could also roll the dough between 2 sheets of parchment paper and place over the marshmallow.

Bake for 25 to 35 minutes, until lightly browned. Cool completely before cutting into bars.