Preheat your oven to 350°F (175°C). Grease your 9x13 inch baking dish or line it with parchment paper for easy removal.
In a large mixing bowl, combine the melted butter, light brown sugar, and creamy peanut butter. Use a whisk to mix until the mixture is smooth and well combined. Add in the eggs and vanilla extract, and whisk again until fully incorporated.
In another bowl, whisk together the all-purpose flour, graham cracker crumbs, old-fashioned oats, cornstarch, baking powder, and salt. Ensure that everything is evenly mixed.
Gradually add the dry ingredient mixture to the wet ingredients, stirring gently with a spatula or wooden spoon until just combined. Be careful not to overmix!
Gently fold in the Reeses peanut butter baking chips and chopped Reese's miniatures. This will add delicious pockets of chocolate and peanut butter throughout your bars.
Pour half of the cookie batter into the prepared baking dish, using a spatula to spread it evenly. Place the Hersheys chocolate bars on top of the batter, ensuring they cover the entire layer. Spread the marshmallow fluff over the chocolate layer, then top with the remaining cookie batter, spreading it out evenly.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs. Be careful not to overbake, as you want these to remain chewy and gooey.
Once baked, remove from the oven and let the bars cool in the pan for at least 20 minutes. Once cooled, cut them into squares and enjoy the delightful combination of flavors!
