Preheat oven to 350 degrees.
Line a 9x13 inch baking dish with parchment paper.
Cream butter and sugar together in a large mixing bowl. Add peanut butter and continue mixing.
Add eggs one at a time, mixing well after each egg. Add in vanilla.
Combine flour, graham cracker crumbs, oats, cornstarch, baking powder, and salt in a small mixing bowl.
Add the flour mixture to the butter mixture, slowly stirring until well combined.
Stir in the peanut butter chips.
Refrigerate cookie dough for at least 30 minutes.
Spread 2/3 of the dough on the bottom of the pan.
Top with Hershey's bars, breaking apart as needed.
Spread marshmallow fluff on top of Hershey's bars.
Stir chopped Reese's into remaining cookie dough.
Drop the remaining dough on top by the teaspoon. It doesn't have to be perfect and it won't cover all the marshmallow fluff.
Bake for 25-30 minutes. Let cool completely and then cut into bars.