Preheat oven to 325°F.
In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, beat the butter and sugars until fluffy and light in color, about 1 minute.
Add in eggs and vanilla. (Scrape down the sides and bottom of the bowl as needed.)
Add in flour, cornstarch, baking soda and salt together until combined. On low speed, slowly mix into the wet ingredients until combined. (dough will be thick)
Add the chocolate chips and M&M’S® and stir with a spoon until incorporated.
Spread half the cookie dough evenly into prepared (lightly greased) skillet.
Pour caramel bits and marshmallow topping evenly over cookie dough.
Using your hands, take a small portion of the remaining cookie dough and flatten. Lay the flattened piece on to of the caramel and marshmallow. Continue this until all the dough is gone. Once you have covered the caramel and marshmallow entirely, take your spoon and gently flatten out the cookie dough in the skillet. (see video for detailed instructions)
Bake for 30-35 minutes or lightly browned on sides.
Allow the cookie to slightly cool in the skillet. (1-2 minutes) Best served hot.
Top with ice cream, hot fudge, and sprinkles if you would like.
To store: Remove cookie from skillet and place in an airtight container. Stays good about 3 days. To help it stay fresh longer, add a piece of bread to the container.