In a bowl, whisk cold butter until slightly creamy
Add sugar and whisk until combined and fluffy
Mix in egg, egg yolk, and vanilla
Add chocolate chips and mix
Add flour, baking powder, baking soda, and salt. Mix gently until just combined (don't overmix)
Cover and chill the dough overnight (or bake immediately if needed)
Let chilled dough sit 15–20 mins, then press into an 8×8 tray
Bake at 175°C (350°F) for 30 to 35 mins until edges are golden and center is soft
Cool before slicing
