Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats to prevent sticking and over-browning.
In a medium bowl, whisk together the flour, cornstarch, baking soda, baking powder, and fine sea salt until well combined. Set aside.
2 1/4 cups all-purpose flour, 2 teaspoons cornstarch, 1/2 teaspoon baking soda, 1/2 teaspoon baking powder, 1/2 teaspoon fine sea salt
In a large mixing bowl, beat the softened butter, light brown sugar, and granulated sugar with a stand mixer (paddle attachment) or hand mixer on medium speed for 2–3 minutes, until light and fluffy. Scrape down the bowl as needed.
3/4 cup unsalted butter, 3/4 cup light brown sugar, 1/2 cup granulated sugar
Beat in the egg, egg yolk, and vanilla extract on medium speed just until smooth and combined. Do not overmix.
1 large egg, 1 large egg yolk, 2 teaspoons pure vanilla extract
Turn the mixer to low and gradually add the dry ingredient mixture. Mix just until no streaks of flour remain. The dough will be thick. If it looks very dry and crumbly, mix in 1 tablespoon of milk, adding up to 1 more tablespoon only if needed to achieve a soft, scoopable dough.
2 1/4 cups all-purpose flour, 2 teaspoons cornstarch, 1/2 teaspoon baking soda, 1/2 teaspoon baking powder, 1/2 teaspoon fine sea salt, 1–2 tablespoons milk
Using a spatula, gently fold in 1 cup of the red and green M&M’s and all of the semi-sweet chocolate chips, reserving about 1/4 cup of M&M’s for topping the cookies. Fold by hand to avoid breaking the candies.
1 1/4 cups red and green M&M’s, 1/2 cup semi-sweet chocolate chips
Cover the bowl and chill the dough for 20–30 minutes to help the cookies stay thick and prevent excessive spreading. If chilling longer (up to 24 hours), let the dough sit at room temperature for 10–15 minutes before scooping.
Using a 1 1/2-tablespoon cookie scoop or a rounded tablespoon, scoop dough onto the prepared baking sheets, spacing about 2 inches apart. Roll each portion gently between your palms for smoother edges. Press a few of the reserved M&M’s onto the top of each dough ball.
1 1/4 cups red and green M&M’s
Bake one sheet at a time for 9–11 minutes, until the edges are set and lightly golden while the centers still look slightly soft and pale. For softer cookies, remove closer to 9 minutes. The cookies will continue to set as they cool.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. For a bakery-style round shape, use a round cookie cutter or glass to gently scoot the hot cookies into circles immediately after they come out of the oven.