Preheat your fan oven to 160°C (325°F) or 175°C (350°F) if you're not using a fan oven. Line your baking sheet with parchment paper.
In a medium sized bowl, sift together flour, cocoa powder, baking powder, baking soda and salt. Set aside.
In a bowl of a standing mixer, beat together softened butter and sugars. It doesn't need to be extra pale and fluffy, just combine them. Add eggs, on at the time, vanilla extract and beat until everything is nicely incorporated.
Fold in your dry ingredients, either on lowest speed of your stand mixer or with a spatula. It should all come together into a soft cookie dough. Add finely chopped white chocolate chunks or chips.
Using an ice cream scoop, scoop balls of dough onto your prepared baking sheet, leaving couple of inches in between them, so they have room to spread. Bake for 10-12 minutes. They're supposed to be a little crispy on the outside, but soft in the middle.
Leave them to cool on the tray for 2 minutes, before transferring them to a cooling rack to cool completely. Store in an airtight container for prolonged freshness.
Notes
If you don't have a stand mixer, you can use a hand mixer or a wooden spoon.
If you're not a fan of white chocolate, you can definitely leave them out or substitute milk or dark chocolate for extra chocolatiness.
Not all ovens are the same. I suggest baking one cookie as a test run for the rest, to determine the best time. :)
If you still don’t own one, I advise you to buy a kitchen scale – they’re cheap and easy to use, and always guarantee the same results in baking!