Preheat oven to 350 degrees and line two baking sheets with parchment paper. In a medium bowl, combine flour, baking soda, cornstarch, and salt until well mixed.
In the bowl of your stand mixer (or a large bowl with a hand mixer) beat butter and both sugars until light and fluffy, approx 3-5 minutes. Scrape bowl as needed.
Beat in egg and egg yolk, followed by vanilla, until well incorporated. Slowly mix flour mixture into butter mixture, scraping down the sides of the bowl as needed. Fold in m&m candies.
Scoop out rounded tablespoonfuls of dough and place at least 2 inches apart on prepared baking sheets. Bake at 350 for 12-14 minutes or until edges are set/just beginning to brown. Let cool on pan for 3-5 minutes before carefully removing to wire rack to cool completely.