Preheat oven to 325 degrees Fahrenheit. Line a 9x13-inch pan with aluminum foil and spray with cooking spray. Set aside.
In a large mixing bowl or stand mixer, cream the butter, brown sugar, and granulated sugar together until light and fluffy, about 5 minutes. Add the eggs and vanilla extract until everything is well incorporated.
Whisk flour, baking soda, cornstarch, and salt in a medium bowl. Add the dry ingredients to the butter/sugar mixture and stir until just combined.
Stir in one cup each of M&M's and chocolate chips.
Spread the batter evenly into the pan and top with remaining M&M's and chocolate chips.
Bake for 30-35 minutes or until the edges are golden brown and have pulled away from the sides.
Allow bars to completely cool and then slice and serve. Enjoy!