To Prep: Preheat the oven to 350 degrees Fahrenheit. Line a 9x11-inch baking dish with parchment paper.
Sift together the flour, cornstarch, baking powder, baking soda, and salt in a large mixing bowl.
In a separate bowl, add the slightly cooled browned butter, brown sugar and white sugar and whisk together until combined. Add the eggs and vanilla and whisk until smooth.
Pour the wet ingredients into the bowl of dry ingredients and fold together with a rubber spatula. The cookie dough should be pretty thick.
Fold the chocolate chips and mini M&M's into the dough until evenly distributed. Scoop the cookie dough into the prepared pan, spreading evenly.
Bake for 24 minutes, or until the top starts to crumble. As soon as you remove the pan from the oven, carefully press extra M&Ms into the top of the cookie bars.
Video
Notes
I like my cookies and cookie bars to be soft-baked, but if you prefer a more cake-like texture you can increase the cooking time for a couple minutes. Be sure to keep an eye on them as they bake so the bars don't cook too much.