Preheat oven to 350 F/177 C. Grease an 8 x 8 square baking pan.
Run graham crackers through a food processor or smash them by hand until they're a fine crumb. Set aside.
In a mixing bowl, whip butter and both sugars. Then add vanilla and egg, and run your mixer until the batter is light and fluffy.
To the mixing bowl, add flour, graham cracker crumbs, baking powder, and salt. Mix until the dough is well formed and no longer sticking to the walls of the bowl.
Split the dough in half, and press one half into the bottom of your prepared baking pan. Layer the chocolate on top, then spread the marshmallow creme over the chocolate, covering the whole surface to the sides of the pan. With the remaining dough, flatten pieces of it and place on top of the marshmallow, shaping it to cover the entire surface.
Bake at 350 F/177 C for 25-30 minutes.
Remove from oven, and let it cool completely before slicing. (I understand the torture of that last bit, but trust me, it'll slice so much better and will be totally worth the wait. Promise!)