Preparation Steps

• In a medium bowl, whisk together the flour, cornstarch, baking soda, salt, and espresso powder (if using). Set aside.
• In a large mixing bowl, whisk the cooled melted butter with the granulated and brown sugars for 60 seconds. Let it stand for 2 minutes, then whisk again for 30 seconds until slightly thickened and glossy.
• Whisk in the egg, egg yolk, and vanilla until smooth and shiny.
• Switch to a spatula and fold the dry mixture into the wet just until no dry streaks remain. Fold in most of the M&Ms and chocolate chips, reserving about 2 tablespoons of each for topping later.
• Cover the bowl and refrigerate the dough for 20 minutes to firm slightly. Meanwhile, preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
• Use a 1/4-cup scoop (about 60 g) to portion the dough. Roll each portion into a tall mound and place 6 per sheet, spacing at least 3 inches apart. Gently press the tops to about 3/4-inch thickness for even baking.
• Bake for 7 minutes, then quickly open the oven and press a few of the reserved M&Ms and chips onto the tops of each cookie. Rap the baking sheet once against the rack to help set the edges, then continue baking 5 to 7 minutes more, until the edges are set and the centers look puffed and matte (not glossy).
• Cool on the baking sheet for 10 minutes. If using, sprinkle turbinado sugar over the warm cookies for a sparkly finish, then transfer to a rack to cool for at least 20 more minutes before serving.
• Practical tip: If your kitchen is warm and the dough softens while scooping, pop the portioned dough on the sheet into the fridge for 5 minutes to keep the cookies thick and prevent overspreading.