Place the butter, granulated sugar and brown sugar in the bowl of a mixer. Beat for 2-3 minutes or until light and fluffy.
Add the eggs and vanilla extract to the bowl. Beat until thoroughly mixed.
In a medium bowl, mix together the flour, cornstarch, baking soda, baking powder and salt. Add half the dry mixture to the mixing bowl and mix on low speed.
Add the remaining dry ingredients and mix on low speed until just combined. Fold in 2 cups of M&M's.
Scoop out 2 tablespoons of dough and roll it into a ball. Repeat with remaining dough.
Place the cookies 2 inches apart on two sheet pans lined with parchment paper or silicone baking mats. Press extra M&M's into the tops of the cookies. Freeze for 10-15 minutes or until firm.
Preheat the oven to 350 degrees F.
Bake one sheet of cookies at a time for 10-12 minutes or until edges are lightly browned.
Let the cookies rest for 3-5 minutes, then transfer to a cooling rack. Serve when cooled, or store in an airtight container for up to 5 days.