Preheat the oven to 350 degrees. Line a 8x8 inch pan with aluminum foil or parchment paper.
In a food processor, pulse the graham cracker sheets until finely ground without any lumps. Combine it with the flour and salt and set aside.
Cream the butter and brown sugar until light and fluffy. Add the vanilla. Add the graham cracker and flour mixture and mix until incorporated.
Press 2/3 of the dough into the bottom of your 8x8 inch pan. Spread the nutella evenly across the bottom. Using a small spoon dollop the marshmallow fluff evenly on top of the nutella. Crumble the remaining dough on top. I would kind of pat the dough flat with my hands and lay it on top and spread it on top. Some of the marshmallow will peek through.
Bake for 30-35 minutes until the top is golden brown and toothpick comes out clean. Let them completely cool before cutting. They can still be a little warm and are amazing!