Preheat oven to 350F. Grease a 10-inch cast iron skillet with butter or cooking spray.

In a medium bowl, combine gluten-free flour blend, cocoa powder, baking soda, and salt. Whisk to combine. Set aside. 

In a large bowl, cream together butter, peanut butter and granulated sugar until light and fluffy. Add egg and vanilla extract and mix to combine. Gradually add in the flour mixture, stirring in between, just until combined. Stir in the chocolate chips. 

Lightly press dough into the cast iron skillet.

Bake at 350F for 20-25 minutes, until edges are set and middle is mostly cooked. Remove from oven at let sit for at least 5 minutes. 

Drizzle melted peanut butter and chocolate over the cookie skillet. Top with ice cream if you wish and serve warm.