Preheat oven to 350 degrees.
Line a 13x9 baking dish with parchment paper, so the paper hangs over the sides a little. Set aside.
In a mixing bowl, whisk together the graham cracker crumbs, flour, baking powder, and salt and set aside.
In a separate large mixing bowl, cream together the butter and sugars with an electric mixer for about 2 minutes. Add in the eggs and vanilla and mix again until smooth. Add the dry mixture to the butter/sugar mixture and combine using the electric mixer until it all comes together into a dough. Divide the dough into 2 portions. Take one of the portions of dough and spread it into the bottom of the prepared baking dish until it meets all of the edges.
Place the chocolate bars down over the dough in one layer. Spread the marshmallow fluff over the chocolate bars evenly.
Lightly flour a board or counter top and smoosh or roll with a rolling pin, the other half of the s'mores cookie dough and make it/roll it the size of the your pan. Slice the dough into strips and lay each piece on top of the fluff until you've covered it all as good as you can. Try to cover up all the marshmallow as good as you can, but dont worry about some marshmallow poking out.
Bake for about 30-35 minutes, until the top is golden brown.
Cool completely before slicing and serving.