Preheat oven to 350 degrees. Lightly grease a 9" square baking pan.

In a stand mixer, beat melted butter and brown sugar until smooth. Add in egg and vanilla and beat until well blended.

Gradually beat in the flour and salt until combined. Stir in graham cracker crumbs.

Spread half of the dough on the bottom of the baking pan. Press peanut butter cups into the dough and spread the marshmallow fluff on top (as carefully as you can, I recommend dipping your spoon in warm water before scooping out the fluff).

Press out the remaining dough on a non-stick surface into the shape of an 8×8 square and place that on top of the fluff, pressing down lightly.

Bake for 20 to 25 minutes. Allow to cool for a couple hours before cutting to give the bars a chance to set.