Line a small baking sheet with parchment paper. Set aside.

Cream butter and sugars together in a large mixing bowl. Beat in the egg and vanilla until thoroughly combined.

Whisk together flour, cornstarch, baking soda, and salt in a small bowl.

Gradually add flour mixture to wet ingredients, mixing just until combined.

Fold in chocolate chips and M&Ms.

Scoop out about 2 tablespoons of cookie dough and roll into balls.

Place cookie dough balls on the prepared baking sheet and repeat until all dough has been used. Press a few M&Ms onto the top of the cookies if desired.

Cover the cookie dough balls with plastic wrap and freeze for at least an hour and up to overnight.

Preheat oven to 350°F.

Place cookie dough balls on a parchment lined baking sheet and bake for 10 to 12 minutes.

Let cookies cool for several minutes on baking sheet before transferring to a cooling rack.

Store in an airtight container for up to a week.