In a small mixing bowl, combine the flour, baking soda, cornstarch, and salt with a whisk. Set aside.
2⅓ cups all-purpose flour, 1 teaspoon baking soda, 1½ teaspoons cornstarch, ½ teaspoon kosher salt
In a medium mixing bowl with a hand mixer, beat the butter, brown sugar, and granulated sugar.
¾ cup unsalted butter, ¾ cup light brown sugar, ½ cup granulated sugar
Add the eggs and vanilla. Mix until combined.
2 large eggs, 2 teaspoons pure vanilla extract
Add the dry mixture, and mix with a large spoon or rubber spatula until completely combined.
Fold in the chocolate chips and M&M's.
½ cup semisweet chocolate chips, 10 tablespoons red and green M&M's
Cover and chill for at least 1 hour to prevent the cookies from spreading.
Preheat oven to 325°F and line a large baking sheet with parchment paper.
Use a cookie scoop to measure 3 tablespoons of dough. Roll into a smooth ball. Place 6 on each baking sheet.
Bake for 12-14 minutes, or until the edges begin to brown.
While the cookies are still warm, press in the remaining M&M's in (just for looks).
Allow the cookies to cool for 10 minutes before transferring to a wire cooling rack.