Preheat oven to 350 degrees.

Line two baking sheets with silicone baking mats or parchment paper.

In the bowl of a stand mixer, beat together the butter and both sugars for about 1 minute. Add egg and vanilla, scrape down bowl and beat for an additional 30 seconds.

In a clean bowl, whisk together the flour, cornstarch, baking soda and salt.

Add the flour mixture to the butter mixture and mix on low speed until flour is just combined.

Set aside 45 mega m&m's. Add the rest of the m&m's to the cookie dough. Mix on low speed until m&m's are mixed into the dough.

Using a large OXO cookie scoop (3 tablespoon capacity), scoop 6 cookies onto each baking sheet. Lightly flatten the tops of the scoops with the palm of your hand and place three m&m's on top of each unbaked cookie.

Bake for 10 minutes, rotating pans halfway through baking time.

Allow cookies to cool on pan for at least 10 minutes before removing to cooling rack to finish cooling. Makes 15 cookies.