Prepare 2 baking sheets with parchment paper, set aside.
Add butter and both sugars to a bowl of a stand mixer fitted with a paddle attachment and beat until creamy, about 3-4 minutes.
Add eggs and vanilla to butter mixture and beat on low-medium until well combined, about 2 minutes.
Add flour, cornstarch, baking powder, baking soda, and salt to a separate mixing bowl, and whisk together.
Add the flour mixture to the butter mixture and mix on low until just combined.
Fold in chocolate chips and M&Ms.
Refrigerate the dough for at least 2-3 hours or overnight.
Preheat oven to 375 degrees F about 20 minutes before removing the cookie dough from the refrigerator.
Roll dough into 10 equal sized balls (roughly around ½ cup each or 140g).
Place several inches apart, cooking about 6 per cookie sheet.
Bake for 15-16 minutes or until beginning to brown on top.
Remove from oven, leave on pan for 15-20 minutes (or until nearly completely cooled) then move to a cooling rack.
Serve warm.