In a small saucepan, melt butter over medium heat and stirring constantly with a wooden spatula. Over the next seven minutes or so, the butter will foam, release a nutty aroma then begin to brown. To avoid burning, remove from heat as soon as brown bits form on the bottom of the pan and set aside to cool.
After browned butter has cooled, add to a large mixing bowl with brown sugar and granulated sugar then beat with an electric mixer until well combined.
Add eggs and egg yolk, one at at time, beating after each until well combined. Scrape down the sides of the bowl with a spatula as needed.
Add vanilla then beat until combined. Set aside.
In a separate bowl, sift together salt, baking soda and flour.
Slowly add dry ingredients to wet, beating on low until just combined. Do not overmix.
Add 1/2 cup of M&Ms and 1/2 cup of chocolate chips to the batter. Mix with wooden spoon until evenly distributed.
Cover bowl with plastic wrap and refrigerate for 30 minutes.
Preheat oven to 350 degrees then grease your 10-inch cast iron skillet.
Using your fingers, press chilled dough into skillet in an even layer. Note: batter will be thick.
Top with remaining chocolate chips and bake for 30 to 35 minutes or until edges are set and slightly golden and center is still soft. Avoid overbaking.
Remove skillet from oven and gently press remaining M&M's into the top of the cookie.
Allow skillet cookie to cool for 10 minutes on a wire rack before slicing and serving.
Cover and store leftovers in the refrigerator up to 3 days.
