Combine the dry ingredients in a bowl: all-purpose flour, cocoa powder, baking, baking soda, cornstarch, and salt.
In a stand mixer (or a bowl with an electric mixer), add the softened butter*, and both the sugars and cream together on high speed for 3-5 minutes until well combined. The mixture should be nice and creamy. Make sure not to overmix at this point!*
Add the room-temperature egg* and vanilla essence into the butter mixture and whisk together until well combined. Make sure the eggs are fully incorporated into the butter and there is no liquid left in the batter.
With the mixer on slow, add the dry ingredients into the batter and keep mixing on low speed until everything is well incorporated and there are no streaks of flour remaining in the mix. You will have a nice, thick dough at this point.
Add the milk and whisk again until completely incorporated.
Finally, add the chocolate chips to the dough and mix on high speed for a few seconds just to incorporate them in, don’t over-mix.
Wrap the prepared cookie dough tightly in cling film and let it chill in the fridge for 2-3 hours, or ideally overnight.