Whisk the flour, baking soda, cornstarch, and salt together in a large bowl. Set aside.

In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until no sugar lumps remain. Whisk in the egg, then the egg egg yolk, followed by the vanilla. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. Fold in the chocolate chips and M&Ms. Cover the dough and chill for at least 2 hours (or up to 3 days).

Take the dough out of the refrigerator and allow to soften slightly at room temperature for 10 minutes.

Preheat the oven to 325 degrees F. Line two large cookie sheets with parchment paper or silicone baking mats.

Roll the dough into balls, about 3 tablespoons of dough per cookie. If the dough is crumbly, the warmth of your hands will help it come together in a ball. Roll the cookie dough balls to be taller rather than wide. Place 8 balls of dough 3 inches apart onto each cookie sheet.

Bake each batch for 11-12 minutes. The cookies will look very soft and underbaked. Remove from the oven and allow to cool on the cookie sheet for 10 minutes before moving to a wire rack to cool completely. The cookies will stay fresh in an airtight container for up to 10 days.