Preheat oven to 325 degrees F.
Mix the flour and baking soda in a bowl and set aside. In another bowl, mix the butter and sugars until they are combined (and look like the first picture above). Add the egg, egg yolk, and vanilla and stir until mixed. Gradually add flour and mix until a dough forms – it will look crumbly at first, but it will come together. I even used my hands to help at one point. Fold in the m&m’s.
Divide the dough how you choose (1/4 cup portions for 18 large cookies or 2 tablespoon portions for 36 smaller cookies). Shape each portion into a ball with your hands then pull each ball into two equal pieces. Turn each half so that the rough side of the half (what used to be the inside of the ball) faces upwards and then squish both halves together. Place the dough rough-side up on a baking sheets. If you’re doing large cookies, I’d leave about 2 inches in between each.
Bake for 12-15 minutes (large cookies) or 8-12 minutes (small cookies) or until the edges are slightly brown. The centers should be soft and puffy. Do not over bake. Let cool completely then dig in!
Note: to melt the butter I simply microwaved it, and to let it cool I allowed it to sit for about 10 minutes.