Preheat the oven to 350 degrees, and line two baking sheets with silicone mats or parchment paper.

In a large bowl, use a hand mixer to cream together the butter, brown sugar, and white sugar until light and fluffy (about 2 to 3 minutes). Add the eggs and vanilla, and mix until combined.

Next, add the flour, baking soda, and salt. Continue to mix on low speed until the ingredients are just combined.

Stir in the chocolate chips and M & Ms by hand. Refrigerate the dough for at least 30 minutes.

Use a cookie scoop, or roll the cookie dough, into golf ball-sized mounds about 2 inches apart. Top with extra M & Ms if using. Bake for 10-12 minutes or until the tops are just barely golden in color.

Remove from the oven, and allow the cookies to cool on the pan for 3-4 minutes before transferring to a wire rack. The centers will be soft.