Using a hand or stand mixer, beat the butter, brown sugar and granulated sugar on medium speed for about 3 minutes, until combined and slightly fluffy.
Add the egg, egg yolks and vanilla. Beat on medium for 1 minute, scraping the bowl once, until smooth.
In a medium bowl, whisk the flour, cornstarch, baking soda, baking powder and salt.
Add the dry ingredients to the mixer bowl in two additions, mixing on low speed until just combined.
Fold in the chocolate by hand with a spatula.
Weigh the dough into 100 g balls (about 3½ oz). Roll each into a tall, rounded ball and place on a plate or tray. Cover and refrigerate for at least 1 hour, or up to 72 hours.
Heat the oven to 350°F (175°C). Line a large baking sheet with parchment paper.
Place 6 chilled dough balls on the sheet, about 3 inches apart. Don’t flatten them. Press a few extra chocolate pieces on the tops.
Bake for 14 to 16 minutes, until the edges are golden and set and the centers look soft and slightly underdone.
While still hot, place a round cutter or glass around each cookie and swirl gently to round the edges (optional).
Sprinkle with flaky salt. Rest the cookies on the sheet for 10 minutes, then move them to a wire rack.
