Preheat oven to 325°F (162°C). Lightly grease your oven-safe skillet.*

In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, beat the butter for 1 minute on medium speed until completely smooth and creamy. Add the granulated sugar and brown sugar and beat on medium high speed until fluffy and light in color. Beat in eggs and vanilla on high speed. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk the flour, cornstarch, baking soda and salt together until combined. On low speed, slowly mix into the wet ingredients until combined. The cookie dough will be quite thick. Add the chocolate chips and M&M’S®, then mix on low for about 5-10 seconds until everything is evenly disbursed.

Using a silicone spatula or wooden spoon, spread the cookie dough evenly into prepared skillet.

Bake for 30-35 minutes or until puffed, browned on the sides, and lightly browned on top. If you want your skillet cookie a little more cooked through, bake for 40 minutes. Allow the cookie to slightly cool in the pan set on a wire rack before digging in. You can scoop/slice and serve or serve with a whole bunch of spoons so everyone can dig in together. I always serve with ice cream and chocolate syrup. Cover leftovers and store in the refrigerator for up to 5 days.