Preheat your oven to 350 degrees and line a baking sheet with parchment paper.
In a medium bowl combine your flour, cornstarch, baking soda and salt. Whisk together until combined and set aside.
In the bowl of a stand mixer with the paddle attachment, mix the softened butter, granulated sugar, brown sugar, egg and vanilla on medium speed until fluffy.
Add in your flour mixture little by little until just combined. Be careful not to overmix.
Stir in 1 1/4 cup of the M&M's with a spatula until evenly distributed.
Optional: you can chill your dough for up to 24 hours, but it is not completely necessary.
Scoop the dough into balls (about 29 grams each if you are weighing them) and place them on your cookie sheet. Press the remaining M&M's into the tops of each cookie dough ball.
Bake for 9-11 minutes or until the edges just start turning golden brown. Allow to cool on the cookie sheet for 5 minutes before transferring to a wire cooling rack.