In a large bowl, whisk together the flour, baking soda, cornstarch and salt. Set aside. In a separate bowl, whisk together the butter and both sugars until smooth. Then whisk in the egg, egg yolks and vanilla. Pour the wet ingredients into the dry and stir until just combined. Then fold in the M&Ms.

Cover the dough tightly with plastic wrap and refrigerate for at least 2 hours. Towards the end of chill time, preheat the oven to 325°F. Line a couple large baking trays with parchment paper or silicone baking mats. Roll the dough into balls, about 1 tbsp of dough per ball and place them on the prepared trays with about 1 inch or so of space between them.

Bake for 6-8 minutes. Cookies should still look very soft and underbaked when they're done, and just starting to brown around the edges, which will result in a soft and chewy cookie once they've cooled. Remove from oven and if you'd like, immediately press in a couple M&Ms gently into the tops of the cookies. Let them cool on the baking tray for 10 minutes, then transfer them to a wire rack to cool completely. They store at room temperature for about a week.