Preheat the oven to 350F (180C). Line a 9x13 inch (23x33 cm) pan with aluminum foil, leaving an overhang around the edges. Alternatively, lightly grease the pan.
In a large bowl, whisk together the melted butter and sugars until you no longer see the butter sitting on top of the sugar. You can also do this using an electric mixer.
Then whisk the eggs and vanilla extract into the butter mixture until you no longer see pieces of eggs. Set aside.
In a separate medium bowl, whisk together the flour, cornstarch, baking soda and salt.
Whisk the dry ingredients into the wet ingredients. Then whisk together until smooth. I typically do a few stirs with a rubber spatula first, then whisk. You could also use an electric mixer. Stop mixing once you no longer see streaks or clumps of flour.
Stir in the chocolate chips and M&Ms.
Pour the batter into the prepared pan and smooth into an even layer.
Bake in the preheated oven for about 25-30 minutes or until the top is golden.
Cool the bars in the pan.
When ready to slice, if you lined the pan with parchment lift the bars out of the pan using the overhang of parchment paper. Place on a cutting board and slice with a sharp knife. If you simply greased the pan, slice the bars in the pan.