Preheat the oven to 350 degrees. Grease an 8-inch square baking pan (I line my pan with foil for easy removal and cutting).
Use an electric mixer to cream together butter and both sugars until light and fluffy. Beat in egg and vanilla. Set aside.
Meanwhile,whisk together flour, graham cracker crumbs, baking powder and salt. Slowly add flour mixture to the creamed butter mixture 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. 2 king-sized Hershey’s bars should fit perfectly side by side. (I had to trim a little off one bar to get mine to fit.)
Spread chocolate with marshmallow creme or fluff. Place blobs of it on the chocolate and then spread it around.
Place remaining dough in a single layer on top of the fluff (I rolled the dough out between two pieces of waxed paper and use a spatula to place pieces of it on the top and pressed the pieces together).
Bake for 30 to 35 minutes (I did mine a little longer to get that nice brown color), until lightly browned. Be sure to cool completely before cutting into bars.