Melt the butter and allow it to cool down to room temperature (it should take around ten minutes).
In a bowl, add the melted butter and sugars, and whisk until combined.
Add the egg and vanilla extract and mix in.
In a separate bowl mix the cornstarch, baking soda, baking powder, salt and flour.
Sift the dry ingredients into the wet and stir together, just until combined (don’t overmix).
Stir in the chocolate chunks/chips, until evenly distributed.
Using your hands or an ice-cream scoop, divide the mixture into five roughly equal parts (mine weighed 120g / 4¼ oz each).
Place the balls on a large baking tray with a lot of space in between them. Pop the tray in the fridge for at least 30 minutes.
Preheat the oven to 325℉ / 165℃.
Bake for 18-23 minutes. You want to just underbake the cookies slightly so that they are soft in the middle. The edges will be browning, but the middle is still soft.
Leave on the tray to cool for a while, then transfer to a cooling rack. They are best eaten warm. If they have completely cooled, pop them in a microwave for 30 seconds for a soft centre. Enjoy!