Preheat oven to 350 degrees F and lightly grease a cast iron skillet
Beat the butter on medium for about 5 min and then add in both sugars.
Stir in egg, vanilla, mascarpone and espresso powder.
Add in the flour, baking soda, and salt; mix well.
Stir in the chocolate chips.
Spread cookie dough into a 6-8 inch cast iron skillet.
Take a small spoon and dollop the peanut butter around the cookie (I added around 3 spoonful)
Bake for 20-25 minutes, or until golden brown and edges are set. Every oven is different! Start checking it after 20 min. The middle should barely be cooked. Once you take it out of the oven, it continues cooking so remember that!
Enjoy with some ice cream and extra toppings like nuts, caramel or chocolate sauce!