Preheat the oven to 350 degrees Fahrenheit.
Line a 9x13 inch aluminum cake pan with foil, leaving overhang on the sides; spray the foil with nonstick cooking spray.
In a stand mixer bowl, beat butter, granulated sugar, and brown sugar together until fluffy.
Add eggs and vanilla extract to the bowl and continue beating until well combined.
Reduce mixer speed and add flour, baking soda, and salt; mix until just combined.
Stir in M&M's and mini chocolate chips, reserving some for topping.
Transfer the dough to the prepared pan and press into an even layer.
Press reserved M&M's and mini chocolate chips on top of the dough.
Bake on the middle oven rack for 35 to 40 minutes.
Allow the bars to cool completely in the pan before lifting out using the foil overhang and cutting into pieces.