¾cup(170 g or 1 1/2 sticks) unsalted butter, melted and slightly cooled
1cup(200 g) packed light brown sugar
½cup(100 g) granulated sugar
1large egg, room temperature
1egg yolk, room temperature
2teaspoonspure vanilla extract
In a medium bowl, whisk together the flour, cornstarch, baking soda and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat on medium speed the butter, brown sugar and granulated sugar until light and fluffy, about 4 minutes. Scrape down the sides of the bowl when necessary. Add the egg, egg yolk and vanilla. Beat until combined, 20 to 30 seconds.
With the mixer on low, slowly add the flour mixture to the dough. Careful not to over beat. With a wooden spoon or rubber spatula, stir in the M&M’s by hand.
Cover cookie dough with plastic wrap and chill for at least 2 hours (or overnight).
When ready to bake, preheat oven to 350° F. Line a baking sheet with parchment paper and set aside.
Use a medium cookie scoop to scoop a heaping tablespoons of the chilled cookie dough. Shape it into a ball, and place onto the prepared baking sheet. Repeat placing each ball of cookie dough about 2-inches apart. Gently press down the balls of cookies with the palm of your hand so the tops are flat. Add sprinkles on top.
Bake for about 12 minutes or until the edges start to turn golden brown. Cool on the pan for 5 minutes and then transfer to a wire rack to cool completely. Store in an airtight container at room temperature.
Chilling dough: If chilling dough for more than 2 hours, let the dough rest on the counter for 30 to 45 minutes or until dough is soft enough to scoop, but still cool to the touch.Store cookies in an airtight container at room temperature for up to 5 days.Freeze cookie dough for up to 3 months. Defrost in the refrigerator overnight before scooping and baking.Recommended tools: Kitchen scale, stand mixer or hand mixer, medium cookie scoop, baking sheet