Preheat oven to 350°F. Grease an 9x12-inch square baking pan.

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. This dough can be hard to spread, so I use a butter knife dipped in warm water or the ball of my hand dipped in warm water. It spreads so much better if you do this.

Place chocolate bars over dough, breaking the chocolate (if necessary) to get it to fit in a single layer no more than 1/4 inch thick.

Spread chocolate with marshmallow creme or fluff.

Place remaining dough in a single layer on top of the fluff. If you put the dough evenly onto the bars in spoonfuls then use a wet knife to spread it, it'll be much easier. Be patient. This part can be tricky. Just go slow and keep wetting your knife to spread the dough evenly.

Bake for 30 to 35 minutes, until lightly browned. Cool completely before cutting into bars.