Preheat the oven to 175℃. Line two or three large sheet trays.
In a large bowl, or the bowl of a stand mixer, beat the cold butter on high for about 3 minutes. Add in both sugars and cream together for another 3 minutes.
Add in the eggs and vanilla and combine again.
Fold in the flour, cocoa powder, cornstarch, baking soda, chocolate chunks and a pinch of salt. Do this until only just combined.
Scoop the dough in approx 100g sized balls onto the baking trays, allowing several inches of space in between.
Bake for 12-14 minutes. Remove and leave on the trays to cool for 5-10 minutes to firm up before eating!