Preheat the oven to 350℉. Butter a 10-inch skillet.
To a large bowl, add the melted butter. Use a wooden spoon to stir in the sugar and cocoa powder until smooth. Stir in the eggs, vanilla extract and salt. Slowly mix in the flour. The batter will be thick.
Spread the batter into the prepared pan. Bake for 8 minutes while you prepare the cookie dough layer.
COOKIE DOUGH LAYER:
In a large bowl, stir together the melted butter, brown sugar and white sugar. Stir in the egg yolk, vanilla extract, baking soda and salt. Slowly stir in the flour. Stir in the chocolate chips.
Scoop tablespoons of dough and flatten between your (clean) fingers, then drop evenly over the brownie layer in the pan. It's okay if some of the brownie shows through. Sprinkle with M&M's.
Bake for an additional 23 to 28 minutes or until the cookie is golden on top. The brownie will stay super fudgy. This is best served on a plate with a fork and vanilla ice cream!