Line 9x13 inch baking dish with aluminum foil and spray with nonstick spray to make for easier removal.
Preheat oven to 350°.
Prepare ingredients, ensuring you have all necessary items.
Cream together butter, sugar, and brown sugar until fluffy.
Using a mixer is the easiest way to cream these ingredients, be sure to start at slow speed and increase to limit the mess.
Once creamed, add in eggs and vanilla extract, and continue mixing.
Add in flour, baking soda, and salt and continue to mix at low speed until combined.
Fold in M&M's and chocolate chips, being sure to reserve some for topping.
Transfer dough to prepared baking dish.
Press additional M&M's and chocolate chips to the surface of doughs.
Bake 35-40 minutes or until golden brown.
Let cool completely and cut into squares.