In a large bowl or stand mixer, 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.
Spread marshmallow fluff evenly over the chocolate layer.
Place remaining dough in a single layer on top of the creme. This is most easily done by putting the second half of the dough in a gallon size freezer bag. Use your palms or rolling pin to flatten it out, and then use scissors to cut down both long sides of the bag, so it will open up book-style. Open it up carefully, and the dough will stick on one side of the bag. Then place the bag, dough side down, on the other three layers. From there peel the bag up and carefully spread the dough where it is uneven
Bake for 30 to 35 minutes, until lightly browned. Cool completely or almost completely for the melty factor before cutting into bars.