Preheat oven to 350 degrees. Line a 9x13 aluminum pan with foil, leave some overhang to make cutting easier. Spray foil generously with nonstick cooking spray.
In the bowl of a stand mixer, beat butter, granulated sugar, and brown sugar together until fluffy. Add in eggs and vanilla extract and continue to beat.
Lower speed of mixer and add flour, baking soda and salt, and mix until combined. Add M&M's and mini chocolate chips and give a final stir, reserving some M&M's and mini chocolate chips for topping cookie bars.
Transfer cookie dough to prepared baking dish. Press additional M&M'S and mini chocolate chips into top of cookie dough.
Bake 30 to 35 minutes. Let cool completely and cut.