In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt until well combined. Set aside.
2 1/4 cups all-purpose flour, 2 teaspoons cornstarch, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt
In a large mixing bowl or stand mixer, beat the softened butter, brown sugar, and granulated sugar on medium speed for 2–3 minutes, until light, creamy, and slightly paler in color.
3/4 cup unsalted butter, 1 cup light brown sugar, 1/2 cup granulated sugar
Add the egg, egg yolk, and vanilla extract to the creamed mixture. Beat on medium speed for 30–45 seconds, scraping down the sides of the bowl, until smooth and glossy.
1 large egg, 1 egg yolk, 2 teaspoons pure vanilla extract
With the mixer on low speed, gradually add the dry ingredient mixture about 1/2 cup at a time. Mix just until no dry flour remains; do not overmix. The dough will be thick and slightly sticky.
2 1/4 cups all-purpose flour, 2 teaspoons cornstarch, 1/2 teaspoon baking soda, 1/2 teaspoon fine sea salt
Using a spatula or wooden spoon, gently fold in 1 1/4 cups of red and green M&M’s and the semi-sweet chocolate chips (if using) until evenly distributed, trying not to break the candy shells.
1 1/4 cups red and green M&M’s, 1/2 cup semi-sweet chocolate chips
Cover the bowl tightly with plastic wrap or transfer the dough to a covered container. Refrigerate for at least 45–60 minutes, and up to 72 hours, to firm up the dough and improve flavor and texture.
When ready to bake, preheat the oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats.
Using a 1.5-tablespoon cookie scoop or a rounded tablespoon, scoop the chilled dough into balls. Roll lightly between your hands for smooth balls and place on prepared baking sheets about 2 inches apart. Press a few extra M&M’s on top of each dough ball for a festive look.
1 1/4 cups red and green M&M’s
Bake one tray at a time for 9–11 minutes, or until the edges look set and just begin to turn golden while the centers still look slightly underbaked and soft. The cookies will finish setting on the baking sheet as they cool.
Let the cookies cool on the baking sheet for 5–7 minutes, then transfer to a wire rack to cool completely. For perfectly round, bakery-style cookies, gently scoot the hot cookies into shape with a round cutter or glass right after baking. Enjoy once cooled or slightly warm.