Prep your oven and pans: Preheat to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.

Mix dry ingredients: In a medium bowl, whisk flour, baking soda, salt, and cornstarch until combined. Set aside.

Combine sugars and butter: In a large bowl, whisk the melted, cooled butter with brown sugar and granulated sugar until smooth and glossy, about 30–45 seconds.

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

Bring it together: Add the dry ingredients to the wet in two additions. Switch to a spatula and fold just until you no longer see dry flour. Do not overmix.

Stir in chocolate: Fold in the chocolate chips or chunks until evenly distributed. The dough will be soft but scoopable.

Chill (recommended): Cover and refrigerate the dough for 30–45 minutes. This helps the cookies hold shape and boosts flavor. If you’re short on time, you can skip it, but expect a bit more spread.

Portion: Scoop 2-tablespoon mounds (about 40 g each) and place them 2 inches apart on prepared sheets. For bakery-style looks, press a few extra chips on top.

Bake: Bake one sheet at a time for 9–12 minutes. Pull them when edges are set and lightly golden, but centers still look soft and slightly underdone.

Finish and cool: If using, sprinkle a tiny pinch of flaky sea salt on hot cookies. Let them cool on the sheet for 5–7 minutes, then transfer to a rack to finish setting.