Preheat the oven to 350 degrees F. Generously grease a 10 to 12-inch cast iron skillet with coconut oil or butter.
In a large bowl, use an electric mixer to cream together the peanut butter and butter. Beat in the brown sugar, granulated sugar, eggs, egg yolk, and vanilla extract.
In a separate bowl, stir together the oats, flour, baking soda, and salt Stir into the wet mixture. Stir in the chocolate chips and M&M's candy.
Transfer the dough to the prepared skillet and spread it out evenly. Sprinkle a few more chocolate chips and M&M's on top, if you'd like.
Bake until the skillet cookie is golden, puffed and set along the outside, but jiggles a little in the center when you shake the skillet- 22 to 28 minutes. The skillet cookie will set more as it cools. Like brownies, be careful not to over-bake.
Cool slightly before cutting and serving. If desired, serve topped with vanilla ice cream.