Prep your tools. Line two baking sheets with parchment paper. Set your oven rack in the middle position. You’ll bake one sheet at a time for even browning.

Mix dry ingredients. In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cornstarch. Set aside.

Combine sugars and butter. In a large bowl, whisk the melted, cooled butter with the brown sugar and granulated sugar until smooth and glossy, about 45 seconds. This helps dissolve some sugar and builds flavor.

Add eggs and vanilla. Whisk in the egg, then the extra yolk, followed by the vanilla. Mix until the batter looks thick and slightly lighter in color.

Fold in dry ingredients. Switch to a rubber spatula. Add the dry mixture in two additions, folding just until no dry streaks remain. Do not overmix or the cookies will be tough.

Add chocolate. Fold in the chocolate chips and chopped chocolate. The mix of sizes creates both big pools and even bits of chocolate throughout.

Chill the dough. Cover the bowl and chill for at least 30 minutes (1–2 hours is even better). Chilling prevents excess spread and improves flavor.

Preheat the oven. Heat to 350°F (175°C) while your dough chills. A fully preheated oven is key for the right texture.

Portion. Scoop dough into 2-tablespoon mounds (about 35–40 g each) and place 2 inches apart on the lined baking sheet. For bakery-style cookies, use a 3-tablespoon scoop and add 1–2 extra minutes of bake time.

Bake. Bake one sheet at a time for 9–12 minutes, until the edges are set and lightly golden but the centers still look slightly underbaked. They’ll continue to set on the pan.

Finishing touches. If using, sprinkle a tiny pinch of flaky sea salt on top as soon as they come out. Let cookies cool on the sheet for 5 minutes, then transfer to a wire rack.

Shape if desired. For perfectly round cookies, gently “scoot” them right after baking using a round cookie cutter slightly larger than the cookie, swirling to nudge edges in.