In a large bowl using a hand-held mixer or stand mixer with paddle attachment, beat the butter, brown sugar, and granulated sugar together on medium speed until combined and creamy, about 2 minutes. Beat in the egg and vanilla. Scrape down the sides and bottom of the bowl as needed.
In a separate bowl, combine flour, cornstarch, baking soda and salt. Add dry ingredients into the wet ingredients, then beat on low speed until combined. The cookie dough will be slightly thick. Add M&M candies and with a rubber spatula, gently fold into cookie dough. Cover dough tightly plastic wrap and chill for at least 1 hour and up to 2 days. Chilling is optional but always preferred.
Preheat oven to 350°F. Line 2 large baking sheets with parchment paper or silicone baking mats.
Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes. The dough will be slightly crumbly, but will come together when you work the dough with your hands. Roll about ½ tablespoons of dough into balls.
Bake for 10-12 minutes, until barely golden brown around the edges. The cookies will look extremely soft when you remove them from the oven. Cool for 5 minutes on the baking sheet. If the cookies are too puffy, try gently pressing down on them with the back of a spoon. They will slightly deflate as you let them cool. If desired, while the cookies are still warm, press a few extra M&M candies into the tops.
Transfer cookies to a cooling rack to cool completely. Enjoy!
