In a bowl attached to stand mixer or using an electric hand mixer, beat the butter, brown sugar, and sugar on medium speed until light and fluffy, about 2 minutes. Add the egg and vanilla, beating until combined, another 2 minutes.
In a separate bowl, combine the flour, cornstarch, baking soda, and salt. Add the dry mixture to the wet ingredients. Mix just until combined. Fold in the m&m’s using a spatula, remembering to save 1/4 cup for the end after the cookies have baked.
Create 1 or 2 tablespoon sized cookie dough balls. Flatten each slightly. Cover with plastic wrap and refrigerate 2 hours (or up to 72 hours). (The longer the better but if you're in a pinch put them in the freezer for 30 minutes; they just need to be chilled. You can bake without chilling, but the warm dough will cause the cookies to spread).
Preheat oven to 350F degrees. Place dough evenly on parchment lined cookie sheets. Bake for 8-10 minutes, until edges have set and tops are just beginning to set. Do not bake longer than 10 minutes (even though you may not believe they’re done). They will firm as they cool. Top cookies while still warm with reserved m&m’s. Allow to cool for 3 minutes on baking sheets before transferring to wire rack to cool completely.