In a stand mixer fitted with the paddle attachment, cream butter, white sugar, and brown sugar on high speed for 5-6 minutes. Scrape down sides halfway. Mix until lighter in color and fluffy.
Add vanilla, eggs, and egg yolk. Mix on low speed until just combined.
Add in bread flour, baking soda, baking powder, and salt and mix on low speed for 1 minute.
Fold in chopped chocolate and mix briefly on low speed to distribute evenly.
Put a few pieces of reserved chocolate into a scoop, then scoop dough into 4oz balls (about ½ cup) and place on a parchment-lined sheet tray.
Cover and freeze for at least 1 hour, preferably overnight
Place frozen dough balls on a baking sheet, allowing a lot of room in between each cookie.
Sprinkle with flaky salt, and bake for 18-19 minutes at 375°F (190°C)until edges are golden and centers are soft.
Let cool slightly before serving