Prep and line: Heat oven to 350°F (175°C). Line two baking sheets with parchment paper. This keeps cookies from spreading too much and makes cleanup easy.

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

Combine wet ingredients: In a large bowl, whisk melted, slightly cooled butter with brown sugar and granulated sugar until glossy and smooth, about 30–45 seconds. Whisk in the egg, egg yolk, and vanilla until the mixture looks thick and creamy.

Add dry to wet: Use a spatula to fold the dry ingredients into the wet mixture just until a few flour streaks remain. Don’t overmix or you’ll lose that soft, chewy texture.

Stir in chocolate: Fold in the chocolate chips or chunks. The dough will be thick and scoopable.

Optional rest: For thicker cookies and deeper flavor, rest the dough 20–30 minutes at room temperature. Not required, but recommended if you have time.

Scoop: Use a 2-tablespoon scoop (generous) or roll into golf-ball-sized rounds. Space them 2 inches apart on the prepared sheets.

Bake: Bake 9–11 minutes, until edges are set and lightly golden but centers still look soft and slightly underdone. For bakery-style ripples, gently lift the tray a couple inches and let it drop on the oven rack at minute 8, then continue baking.

Finish: Immediately after baking, press a few extra chocolate chips on top for a polished look. Sprinkle with flaky sea salt if you like that sweet-salty finish.

Cool: Let cookies cool on the sheet for 5 minutes, then transfer to a rack. They’ll firm up as they cool, staying soft and chewy inside.