Line a small baking sheet with parchment paper and set aside. This will help prevent the cookies from sticking and make for easy cleanup.
In a large mixing bowl, cream together the unsalted butter, brown sugar, and granulated sugar. Use a hand mixer or whisk until the mixture is light and fluffy. This should take about 2 to 3 minutes.
Add the egg and vanilla extract to the creamed mixture. Beat until everything is thoroughly combined. You want the mixture to be smooth and shiny.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. This ensures even distribution of the leavening agent and prevents any clumps.
Gradually add the flour mixture to the wet ingredients, mixing just until combined. Be careful not to overmix; otherwise, your cookies may turn out tough.
Fold in the semi-sweet chocolate chips and M&Ms gently. The colors should be evenly distributed throughout the dough.
Scoop out about 2 tablespoons of dough and roll them into balls. A cookie scoop makes this step easier and ensures uniform-sized cookies.
Place the cookie dough balls on the prepared baking sheet, leaving space between each for spreading. Press a few extra M&Ms onto the tops of each cookie if desired.
Cover the cookie dough balls with plastic wrap and freeze for at least an hour, or up to overnight. This step helps the cookies maintain their shape while baking.
Preheat your oven to 350°F (175°C). This temperature is perfect for baking cookies, allowing them to rise and become golden brown.
Once the oven is preheated, place the cookie dough balls on a parchment-lined baking sheet. Bake for 10 to 12 minutes. They should look slightly under-baked when you take them out.
Let the cookies cool on the baking sheet for several minutes before transferring them to a cooling rack. This allows them to set and firm up a bit.
Store the cookies in an airtight container for up to a week, although I doubt they'll last that long!
