Preheat your oven to 350°F (175°C) and prepare an 8x11 inch baking pan by lining it with parchment paper.

In a large mixing bowl, combine softened unsalted butter and packed brown sugar. Beat until light and fluffy, about 2-3 minutes. Then add the egg, regular peanut butter, and vanilla extract, mixing until well-combined. Gradually fold in all-purpose flour and chocolate chips.

Press about 2/3 of the cookie dough into the prepared baking pan to form a solid crust.

Layer the chopped peanut butter cups evenly over the pressed cookie dough, ensuring they are well distributed.

Spoon marshmallow fluff over the peanut butter cups, spreading it evenly.

Crumble the reserved cookie dough over the marshmallow fluff, covering it entirely.

Bake the Fluffernutter Bars in the preheated oven for about 25 minutes, ensuring the edges turn golden brown while the center remains soft.

Allow the bars to cool in the pan for one hour before lifting them out and transferring to a cooling rack.