Preheat oven to 350 degrees F with rack in the middle position. Lightly butter 2, 6-1/2 inch cast iron pans.**
In a bowl, whisk together flour, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl with an electric hand mixer, cream butter, brown sugar, and granulated sugar until light and fluffy, about 3-4 minutes.
Beat in peanut butter. Scrape the bowl and add egg and vanilla extract. Mix to combine on medium-low speed.
Scrape the bowl and add the flour mixture. Mix on low speed to combine.
Use a spatula or wooden spoon to gently stir in the chocolate chips and peanut butter cups until distributed throughout the cookie dough.
Divide the mixture between the two buttered pans, spreading the dough evenly with a spoon or spatula. If desired, sprinkle a few extra chocolate chips and/or peanut butter cups over the top. If adding extra peanut butter cups, press them into the dough just slightly.
Bake 20-25 minutes (mine are usually done to our liking in the 24-25 minute range), until the skillets are puffed, golden, and the centers are just set (and still slightly wobbly). (Or, to your desired doneness; increase baking time for a firmer-centered cookie.) Cool 10 minutes before serving with ice cream and drizzles of peanut butter sauce and/or chocolate sauce, if desired.
* I love using the extra small "unwrapped minis" from Reese's product line in this recipe. I've found them either in the baking or candy sections at my local grocery stores. If you're unable to source them or prefer to use Reese's "miniature" peanut butter cups (which are a little bit larger), cut them in half before measuring and adding to the cookie dough.**I've never experienced spillover with this recipe baked in Lodge 6-1/2 inch cast iron skillets; however, I have not tested other skillet brands. The first time you bake them, you might want to place the skillets on a preheated baking sheet to be safe.Don't have small skillets? This recipe can be baked in a single 8- or 10-inch cast iron skillet. Baking time will take closer to 25-30 minutes.