In a stand mixture with the paddle attachment, cream together softened butter and brown sugar until light and fluffy; about 3 minutes.

Add egg and vanilla and stir for 1 minute, until completely combined.

Stir in flour, baking soda and cornstarch, just until combined. I prefer to use a wooden spoon for this step.

Gently fold in chocolate chips and m&ms.

Use a large cookie scoop or ice cream scoop (about 3-4 tablespoons) to shape dough into balls and place on a parchment paper lined cookie sheet. Chill in the refrigerator for 30 minutes.

Preheat oven to 350 degrees. Bake cookies for 10-12 minutes, until the edges just start to brown.

Cool on baking sheets for about 5 minutes before transferring to a wire rack to cool completely.

Store cookies in a tightly sealed container for up to 1 week, or freeze for unto 4 months.