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Whisk the flour, cornstarch, baking soda, and salt together in a large bowl. Set aside.
In large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, beat softened butter for about 1 minute on medium speed until creamy. Add the brown sugar and granulated sugar, then beat on medium-high speed until creamy and combined. Scrape down the sides and bottom of the bowl as needed. Beat in egg, egg yolk, and vanilla extract. Scrape down the sides and bottom of the bowl as needed. With the mixer running on low speed, slowly add dry ingredients to wet ingredients. Pour in the M&Ms and beat on low speed for 20 seconds or until combined. Once completely combined, cover cookie dough tightly and chill in the refrigerator for 30 minutes and up to 5 days.
Preheat oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.
Roll cookie dough into balls, about 1.5 Tablespoons of dough per cookie, and arrange 3 inches apart on the baking sheets. Bake for 11-12 minutes or until lightly browned on the sides. The centers will look soft.
Remove from the oven and allow cookies to cool on the baking sheet for 5 minutes. While the cookies are still warm, feel free to press a couple more M&Ms into the tops. This is just for looks. Transfer to a wire rack to cool completely.
Cookies stay fresh covered at room temperature for up to 1 week.