Preheat your oven to 350°F (175°C). This step is crucial for even baking.
Line your 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal later.
Crush the graham crackers into fine crumbs using a food processor or rolling pin in a zip-top bag. Combine graham cracker crumbs, flour, and salt in a bowl and mix well.
In a separate bowl, beat the softened butter and brown sugar together until light and fluffy, about 2-3 minutes.
Mix in the vanilla extract, then gradually add the dry ingredients to the butter mixture. Stir until just combined; do not overmix.
Press about two-thirds of the graham cracker mixture evenly into the bottom of the prepared baking pan to form the base.
Break the Hershey's chocolate bars into pieces and layer them evenly over the pressed graham cracker base.
Dollop the marshmallow fluff over the chocolate layer and spread it evenly with a spatula, covering the chocolate completely.
Crumble the remaining graham cracker mixture over the marshmallow fluff layer as the topping.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and set. Watch carefully to avoid over-baking.
Remove from oven and let cool in the pan for about 10 minutes. Use the parchment overhang to lift the bars out onto a wire rack and cool completely. Slice into squares and enjoy!