Preheat the oven to 350 F and lightly grease a 10-12 inch oven-safe skillet. Set aside.
In a medium sized bowl, whisk together the flour, baking soda, cornstarch and salt.
In a large bowl, combine the wet ingredients: melted butter, brown sugar, granulated sugar, egg and egg yolk, and vanilla. Whisk together until combined. Now add in the dry ingredient mixture, and mix until just combined. Don't over mix.
Gently fold in the M&M's.
Transfer the cookie dough to the skillet, and press with your hands until it's evenly spread across the skillet.
Bake for about 10-15 minutes, until the edges are light golden and the middle is still slightly soft and undercooked.
Remove the skillet from the oven and let rest for 15-20 minutes (the cookie will continue cooking in the heat of the skillet) and then serve with ice cream.