In a large bowl, combine the butter, brown sugar, and white sugar. Mix until smooth and well combined. Do not cream.
Add the egg, egg yolk, and vanilla, then mix until fully incorporated.
Add the dry ingredients (flour, cornstarch, baking powder, baking soda, and salt) and fold until just combined.
Fold in the chocolate chips or chunks until evenly distributed. Do not overmix.
Divide the dough into 150g portions and shape each into a rough ball.
Place the dough balls on a lined tray, cover, and refrigerate for at least 1 hour or overnight.
Bake in a preheated 190°C (375°F) oven for 13-16 minutes, or until the edges are golden brown and the centers are still slightly soft.
Allow the cookies to cool on the baking tray for a few minutes, then transfer to a wire rack. If desired, sprinkle with flaky sea salt and let cool for about 15 minutes before serving.
