Preheat the oven to 350 degrees and spray an 8 inch square baking dish with cooking spray, or make a foil sling and spray the foil with cooking spray (If you want to take the bars out of the pan to cut and serve I'd use the foil sling. If you plan on serving out of the dish, then you can skip that step).

In a food processor or a blender add the graham crackers and blend until you have made smooth crumbs.

Add the flour and salt and blend to combine.

In a medium bowl or the bowl of your stand mixer add the butter and brown sugar and beat until light and fluffy, about 3 minutes.

Add the vanilla and the egg and beat until well combined. Add the cracker crumb mixture and stir until just incorporated.

Place 2/3 of the dough in the bottom of the prepared dish and press it flat with your hands or the bottom of a greased measuring cup.

Add the Nutella and spread if around to cover the dough. Add the marshmallow fluff and spread it around to cover the Nutella.

Place the remaining dough over the top in small pieces, it's ok if you can see marshmallow fluff poking out.

Bake for 35 to 40 minutes until a toothpick inserted in the center comes out with no cookie dough stuck to it.

Let the bars cool before serving. If you are planning on removing them from the pan to cut and serve let them cool at least 4 hours before cutting.