Whisk the flour, cornstarch, baking soda, fine sea salt, and espresso powder in a medium bowl until evenly combined; set aside.
In a large bowl with a mixer (paddle), beat the softened butter with the brown sugar and granulated sugar on medium-high until pale and fluffy, 3–4 minutes. Scrape the bowl.
Add the egg, egg yolk, and vanilla. Mix on medium just until the mixture looks silky and emulsified, about 30 seconds.
On low speed, add the dry ingredients in two additions, mixing only until a few streaks of flour remain. Fold in the chocolate mix by hand to avoid overworking the dough.
Cover and chill the dough for 30 minutes to firm slightly. While it chills, line two rimmed baking sheets with parchment and position a rack in the center of the oven.
Preheat the oven to 375°F (190°C). Using a large scoop (about 1/3 cup), portion 10 mounds. Stack two mounds together for each cookie to create tall towers and set them 4 inches apart. Press a few extra chocolate pieces on top and sprinkle lightly with flaky sea salt if using.
Bake at 375°F for 7 minutes, then (without opening the door for long) reduce the oven to 350°F and continue baking 5–7 minutes more, until edges are set and centers look puffed with a slight wobble.
Firm up the edges by giving the pan one confident tap on the rack, then cool cookies on the sheet for 10–15 minutes before transferring to a wire rack to finish setting.