Preheat the oven to 350 and prepare a cookie sheet with parchment paper. 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 M&Ms and combine with a rubber spatula.
Form the cookie dough balls in 1 tablespoon size.
Bake the cookie dough for 8-12 minutes. About halfway through, tap the edge of the pan on the oven rack to help flatten the cookies. Take out the cookies when the edges are golden brown and set. Let the cookies cool on the cooling sheet for 10 minutes and continue to cool on a cooling rack.
