These M&M's cookie bars are crisp on the outside, and warm and gooey in the middle. They're loaded with chocolate chips and M&M candies, and so delicious!
Preheat the oven to 350. Line a 9 x 13 baking dish with foil or parchment.
In a large bowl or the bowl of a stand mixer, mix together the butter, brown sugar, and sugar. Add the egg, egg yolk, and vanilla, and mix until combined. Add the flour, baking soda, and salt, then mix to form a soft dough. Gently fold in the chocolate chips and M&M's candies.
Spread the cookie dough in the prepared baking dish, sprinkling with additional chocolate chips and M&M's if desired.
Bake 24-28 minutes, or until the top is light golden brown and a toothpick inserted two inches from the side comes out clean or with a few moist crumbs. Let cool, then slice into squares.