Preheat oven to 350 degrees. Line a 9x13 baking dish 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, mini chocolate chips and white 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, mini chocolate chips and white chocolate chips into top of cookie dough.
Bake 30 to 35 minutes on oven rack in lower third of oven to prevent over browning. Let cool completely and cut.