Preheat your oven to 350°F. Line a 9×13 inch baking dish with parchment paper and set aside. This step is essential for easy removal of the bars later.
In a large mixing bowl, cream together the butter and light brown sugar until well combined and fluffy. This should take about five minutes, and should be light in color.
Add in the creamy peanut butter and continue mixing until the mixture is smooth and fully combined with no lumps.
Crack in the eggs one at a time, mixing well after each addition. The eggs help bind the ingredients together, so make sure they’re well incorporated.
Stir in the vanilla extract for an extra layer of flavor.
In a separate small mixing bowl, combine the all-purpose flour, graham cracker crumbs, old fashioned oats, cornstarch, baking powder, and salt. Whisk these dry ingredients together until they’re evenly mixed.
Gradually add the dry mixture to the wet mixture, stirring slowly until well combined. The dough should come together nicely.
Fold in the Reese’s peanut butter baking chips, evenly distributing them throughout the dough.
Refrigerate the cookie dough for at least 30 minutes. This will help the bars hold their shape while baking.
After chilling, spread 2/3 of the dough evenly on the bottom of your prepared baking pan. Make sure to press it down firmly.
Now, take the Hershey’s chocolate bars and break them apart as needed, spreading them evenly over the cookie dough layer.
Spread the marshmallow fluff generously on top of the chocolate bars, creating a fluffy layer.
Take the reserved cookie dough and stir in the chopped Reese's miniatures. Drop spoonfuls of the dough over the fluff; it doesn’t have to cover everything perfectly.
Bake for 25 to 30 minutes or until the edges are golden brown and the center is set. Let the bars cool completely in the pan.
Once cooled, cut into squares and enjoy your Reese’s Peanut Butter S’mores Oatmeal Cookie Bars!
