Preheat the oven to 350 degrees Fahrenheit. Line an 8x8-inch pan with aluminum foil or parchment paper.
In a food processor, pulse the 9 sheets graham crackers until finely ground without any lumps. Mix the graham cracker crumbs with the 1 cup + 2 tablespoons all-purpose flour and ½ tsp salt and set aside.
Using a hand mixer, whisk the 1 cup softened butter and ⅔ cup brown sugar together until light and fluffy, then add the 1 teaspoon vanilla extract.
Add the graham cracker mixture to the butter mixture and mix until incorporated.
Press 2/3 of the dough mixture into the bottom of your prepared pan. Spread the 1 cup Nutella evenly across the bottom crust.
Using a small spoon dollop the ½ (7-ounce) jar marshmallow fluff and spread it evenly on top of the Nutella layer. Crumble the remaining dough on top.
Lightly pat the dough flat and spread it across the top, letting some marshmallow peek through.
Bake for 30-35 minutes until the top is golden brown and a toothpick comes out clean. Let them cool before cutting.