Preheat oven to 350 degrees and grease a 12 inch cast iron skillet with butter.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar and peanut butter.

Beat in the the vanilla and the eggs, one at a time.

In a separate bowl, whisk (or sift) together the flour, xanthan gum, baking soda and salt.

Add half the dry ingredients to the butter/sugar mixture and mix on low speed until incorporated. Mix in the remaining flour mixture until completely combined. Mix in the peanut butter cups.

Transfer the batter to the prepared skillet. Bake at 350 degrees for 20 minutes. If you're using a 10 inch skillet, increase baking time. Let cool for at least 30 minutes before serving. Cool completely for portable sliceable pieces.