Preheat your oven to 350°F and generously butter a 6" cast iron skillet. You could also use a regular 6" cake pan instead, just adjust the baking time.
Whisk together the flour, baking soda and salt in a bowl and mix well with a whisk until well combined; set aside.
In a separate mixing bowl, combine the granulated sugar, brown sugar and vanilla extract. Add the melted butter and stir with a fork until well combined.
Add the egg yolk, stir well, and then stir in the flour mixture. Mix until completely combined, then stir in the chocolate chips.
Transfer the cookie dough to the cast iron skillet and spread out evenly across the skillet. Add your M&M's by pressing them into the top of the cookie dough.
Bake for about 18-20 minutes, or until the edges are puffed and browned and the center appears set.
Remove from the oven and press a few additional chocolate chips across the surface of the cookie, if desired; allow to cool for about 5 minutes.
Top with a large scoop of vanilla ice cream and enjoy!