Preheat oven to 350°F and grease and flour a 9-inch cast iron skillet.
In the bowl of a stand mixer, cream together butter and cream cheese until smooth. Add peanut butter, both sugars, and vanilla; mix well. Add eggs; mix until incorporated.
Add flour, quick oats, baking powder, baking soda, and salt. Mix until all dry ingredients are incorporated. (Dough will be a little soft.) Add M&Ms and chocolate chunks; mix until evenly distributed throughout.
Spread cookie dough in the prepared skillet. Sprinkle additional M&Ms and chocolate chunks on top.
Bake for 22 to 24 minutes, or until edges are golden brown and cookie is set in the middle. Remove from oven and let cool. Serve warm with ice cream.