In a large bowl, mix together the butter, sugars and vanilla. Add in the eggs one at a time, mixing after each addition.
In a separate bowl, mix together the flour, cornstarch, baking soda and salt.
Slowly mix the dry ingredients into the wet ingredients until fully combined.
Using a medium cookie scoop, scoop the cookie dough onto a parchment paper lined baking sheet.
Fill a shallow bowl with the mini M&Ms. Gently press the cookie dough balls into the M&Ms and return to the baking sheet. Place in the fridge for 30 minutes.
Preheat the oven to 350℉. Place 5 cookie dough balls 2 inches apart onto a parchment paper lined baking sheet (careful, because these cookies spread a lot!). Leave the rest in the fridge until needed.
Bake the cookies in the preheated oven for 11-12 minutes, or until the edges are set. Allow to cool on the baking sheet for 5 minutes before transferring to wire racks to cool completely.