Preheat the oven to 350 degrees Fahrenheit. Line a baking sheet or two with parchment paper or a silicone liner.
Cream together the butter and sugars in a large bowl for about 3 minutes.
Add in the eggs and vanilla and stir into the creamed butter.
Add the dry ingredients (flour, cornstarch, baking soda, and salt) to the wet ingredients and stir everything together till combined, and you have a nice soft cookie dough.
Add in 2 cups of the m&ms and stir them until mixed evenly into the batter.
Scoop the cookie dough into large 2 TBS sized cookie balls. Roll them into smooth balls.
Place cookie balls onto the prepared cookie sheet about 2-3 inches apart to allow for spread.
Top the cookies with a few extra m&ms if desired.
Bake the cookies for 8 to 10 minutes, or until the edges just start to turn golden, and the tops are barely set.
Remove the pan from the oven. Leave the cookies on the baking sheet for 5 to 10 minutes to cool, and continue baking.
Transfer the cookies to a cooling rack to cool completely, or let them cool another 5 to 10 minutes, then enjoy.