In a bowl, whisk together butter, brown sugar, and white sugar until just combined. Mix in the egg and vanilla extract. Add flour, cornstarch, baking powder, and salt. Stir gently – do not overmix. Fold in both kinds of chocolate chips.

Cover and freeze the dough for at least 30 minutes. This helps the cookies hold their shape.

Preheat oven to 200°C (390°F), top and bottom heat. Place the baking tray inside the oven to preheat as well.

Place the cookie dough balls onto the hot tray. Bake on the lower rack at 175°C (350°F) for 7 minutes. Move the tray to the middle rack and continue baking at 180°C (355°F) for another 7–8 minutes, until golden around the edges but still soft in the center.

Let cookies cool on the tray for 5 minutes before transferring to a wire rack.