Preheat the oven to 325°F (163°C) while you prepare your skillet. Grease a 12-inch cast iron skillet generously.

Cream together the softened butter, granulated sugar, and brown sugar in a large mixing bowl until light and fluffy.

Add the eggs one at a time, blending well after each addition, then mix in the vanilla extract.

Sift in flour, baking soda, and salt into the wet mixture and gently mix until just combined.

Fold in the mini chocolate chips and half of the M&Ms. Set aside remaining M&Ms for topping.

Press the dough evenly into the prepared skillet, ensuring edges are slightly higher.

Bake for 25 minutes until edges are golden brown and center is soft and gooey.

Sprinkle remaining M&Ms on top and return the skillet to the oven for an additional 5-8 minutes.

Cool the Skillet Cookie for 30-45 minutes before serving to allow it to set.