Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the unsalted butter, brown sugar, and granulated sugar. Beat until light and fluffy, about 2-3 minutes.
Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined.
Gently fold in the semi-sweet chocolate chunks and M&Ms until evenly distributed.
Using a cookie scoop, portion out the dough onto a baking sheet lined with parchment paper. Leave space between each cookie.
Bake in the preheated oven for 10-12 minutes, or until the edges are golden brown.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring to a wire rack to cool completely.
