Soft & Chewy M&M Cookies

If baking the cookie dough immediately, preheat the oven to 350ºF. Line baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt.
In a large mixing bowl, use an electric stand or hand mixer fitted with the paddle attachment to beat the butter, brown sugar, and granulated sugar on medium-high speed until light and fluffy, about 2 to 3 minutes. Scrape down the sides and bottom of the bowl before adding in the vanilla, eggs, and egg yolk, beating well after each addition, and scraping down the bowl as needed. Slowly beat in the flour mixture. Stir in the M&M candies and chocolate chips with a rubber spatula, reserving about 1/4 cup M&Ms to garnish each cookie dough ball.
For best results if time permits, wrap dough in plastic wrap and refrigerate for at least 24 hours but no more than 72 hours. This will improve both the flavor and texture of the cookies. Let dough sit at room temperature just until it is soft enough to scoop.
Using a large spring-loaded cookie scoop, divide the dough into 3-tablespoon-sized balls and drop onto prepared baking sheets. Press a few of the saved M&M candies into the top of each ball of dough.
Bake at 350°F for about 11 to 13 minutes, or until golden brown but still ever so slightly wet looking on top. Cool for 5 minutes before removing to a cooling rack to cool completely. Cookies can be stored in an airtight container at room temperature for up to 3 days.