Preheat oven to 350°F (175°C). Line a baking tray with parchment paper or a silicone baking mat.

Whisk dry ingredients: In a bowl, mix flour, baking soda, salt (and cornstarch, if using). Set aside.

Cream butter and sugars: In a large bowl, beat together softened butter, brown sugar, and granulated sugar until smooth and fluffy.

Add egg and vanilla: Mix in the egg, additional yolk, and vanilla extract until well combined.

Combine mixtures: Gradually add the dry ingredients into the wet ingredients and stir until just combined.

Fold in chocolate chips. Use a spatula or wooden spoon to incorporate the chocolate chips evenly.

Chill the dough (optional): For thicker, chewier cookies, refrigerate the dough for at least 1 hour or overnight.

Scoop and bake: Scoop 1.5 tbsp-sized dough balls and place them 2 inches apart on the prepared tray.

Bake for 9–11 minutes, or until the edges are lightly golden and the centers look slightly underbaked.

Cool: Let the cookies rest on the tray for 5–10 minutes before transferring to a wire rack.

Optional: Sprinkle warm cookies with flaky sea salt for extra flavor.