In a medium bowl, whisk together flour, baking soda, cornstarch, and salt. Set aside.
In a large bowl, whisk melted (but cooled) butter, brown sugar, and granulated sugar until smooth and lump-free.
Add the whole egg and egg yolk. Whisk until fully combined. Add vanilla or almond extract and whisk again.
Add dry ingredients to the wet mixture. Stir gently with a rubber spatula until just combined.
The dough will be soft and slightly greasy—this is correct.
Fold in chocolate chips. It may feel thick and stubborn—keep folding until evenly distributed.
Cover tightly and refrigerate for 2–3 hours or overnight. This prevents spreading and improves texture.
Remove dough from fridge and let sit at room temperature for 10 minutes to soften slightly.
Preheat oven to 325°F (163°C). Line baking sheets with parchment.
Roll dough into tall balls (do not flatten). Space them well apart.
Bake for 12–13 minutes, until the bottoms are lightly golden. Centers will look underbaked—perfect.
Let cookies cool on the baking sheet for 10 minutes, then transfer to a rack. They finish baking as they cool.
