Preheat the oven to 350ºF degrees and grease a 10-inch cast iron skillet with cooking spray or butter.

In a large bowl using a hand-held mixer or a stand mixer fitted with the paddle attachment, cream the butter, peanut butter, and brown sugar together on high speed until lightened and fluffy. Beat in the egg and vanilla until combined.

Add the flour, baking soda, and salt, and continue to beat on low until mixture forms a dough. Stir in the peanut butter chips.

Press about half of the dough evenly into the cast iron skillet. Spread the marshmallow creme on top. Take chunks of the remaining dough, flatten slightly, and layer them side by side on top of the marshmallow creme. Try to cover as much of the marshmallow creme as possible.

Bake for 20-25 minutes, or until the edges are golden. Allow to cool for at least 20-30 minutes. Serve with a scoop of ice cream, if desired.