Brown Butter Chocolate M&M Oatmeal Cookies

Preheat the oven to 350 degrees (F). Line two large baking sheets with parchment paper and set aside.
In a medium bowl, combine the flour, baking soda, salt, cinnamon, and oats. Set aside until needed. 
In a large saucepan over medium heat, melt the butter. Continue cooking, stirring frequently with a wooden spoon, 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 that have settled on the bottom of the pan, into a large mixing bowl.Whisk in both sugars and the vanilla, and beat well to combine, about 2 minutes. 
Whisk in the eggs and egg yolk, beating until eggs are fully incorporated into the batter.
Fold in the dry ingredients, chocolate chunks, and m&ms, mixing just until everything is combined. Do not chill cookie dough! This cookie dough is best baked right away. 
Using a large spring loaded scoop, scoop about 3 tablespoons of cookie dough (70 grams) into the palm of your hands and roll it into a ball. Place on the prepared baking sheet, and repeat with all cookie dough, leaving 2-inches between each cookie for spreading. 
Bake, one tray at a time, for 11 minutes, or until spread out and lightly golden brown in color. 
As soon as the cookies come out of the oven, use a spatula to gently press in any edges that spread too much while baking. You can also press on some extra M&Ms at this point, if desired. 
Cool on the baking sheet for 25 minutes, then sprinkle with sea salt and enjoy warm, or transfer to a cooling rack to cool completely.