Preheat the oven to 350 degrees. Line two large baking sheets with parchment paper, set aside.
In a medium bowl, combine flour, salt, cinnamon, baking soda and oats. Set aside.
In a large saucepan over medium heat, brown the butter. Stirring frequently with a rubber spatula, until the butter smells nutty and is a light golden brown color with tiny golden specks on the bottom of the pan.
Remove from heat and scrape the butter (and any brown butter bits) into a large mixing bowl. Add in both sugars and the vanilla, and mix well to combine, about 2 minutes.
Mix in the eggs and egg yolk.
Fold in the dry ingredients, chocolate chunks, and m&ms, mixing just until everything is combined. (Don't chill cookie dough! This cookie dough is best baked right away.)
Scoop about 3 tablespoons of cookie dough onto the lined baking sheet. Place the cookies about 2 inches apart.
Bake for 11 minutes, or until spread out and lightly golden brown in color.
As soon as the cookies come out of the oven, press on some extra M&Ms on top, if desired.
Cool on the baking sheet for 10 minutes, then sprinkle with sea salt and enjoy warm, or transfer to a cooling rack to cool completely.
Enjoy!