In a large mixing bowl, add the cold cubed butter and whisk until slightly creamy
Add the white sugar and light brown sugar, then whisk together until well combined and fluffy
Add the egg, egg yolk, vanilla extract, and chocolate chips or chunks. Whisk until fully incorporated
Add the flour, baking powder, baking soda, and salt. Gently mix until just combined, do not overmix
Place the cookie dough into an 8x8 square tray
Preheat oven to 175°C (350°F). Bake for 30-35 minutes or until the tops are lightly golden while the centres remain soft
If the top turns golden but the inside is still very raw, cover loosely with foil and continue baking
Remove from the oven and let cool and set before cutting
Serve warm with ice cream
