Prepare a cookie sheet with parchment paper and set to the side.
In a medium-sized bowl, combine the flour, cornstarch, baking soda, and salt. Set to the side.
In a medium-sized bowl with a hand mixer or in the bowl of a stand mixer with a paddle attachment, cream the butter until fluffy, about 2-3 minutes. Scrape the bowl down and add the sugar, followed by the eggs and vanilla. Continue to scrape down the bowl as needed.
Add in the flour mixture to the butter mixture and mix until combined. Scrape down the bowl as needed. Add in the full bag of M&Ms and combine.
Form the cookie dough balls in 1 tablespoon sized. Place the cookie dough balls on the prepared parchment and cover the cookie sheet with plastic wrap. Chill the cookie dough balls in the fridge for an hour or overnight.
Preheat the oven to 350 and take the cookie dough out of the fridge. Bake the cookie dough for 10-11 minutes, until the edges of the cookies are set. Let the cookies cool on the cooling sheet for 10 minutes and continue to cool on a cooling rack.
