Mix flour, baking soda, baking powder, salt, and cornstarch in a bowl.

Gently melt the butter until just liquid, then let it cool slightly.

Mix melted butter with brown sugar until smooth.

Whisk in the egg, egg yolk, and vanilla extract until combined.

Stir in the dry ingredients until a soft dough forms.

Fold in the chocolate chips.

Refrigerate the dough for at least 30 minutes.

Roll the dough into balls and place on a lined baking tray.

Bake at 180°C (350°F) for 12–14 minutes until lightly golden.

Let cool slightly before serving.