Into a bowl, sift in flour, salt, baking powder and cocoa powder. Mix everything well and set aside.
In another bowl, add in granulated and brown sugar together with butter. Mix until everything is well combined. Then add in vanilla extract and the eggs, 1 by 1, mixing well after each addition.
Add in previously sifted flour mixture and mix just until well combined. Don't over mix to avoid hard, dense cookies. Cover the bowl with a cling film and leave the batter to chill in the fridge overnight. The batter needs to be really firm before using.
Once firmed, scoop the batter and form it into a 1 inch (2.5 cm) ball, more or less. Place it in a bowl containing icing sugar and coat the ball well with it. Place it on a baking tray with a baking sheet, 2 inches (5 cm) apart.
Bake in a preheated oven at 350ºF (175ºC) for 10 minutes. Don't over bake them so avoid them getting dry. Enjoy!
If the balls get too sticky then put the batter back in the fridge to get firm back again before baking. The cookies won't rise properly if they're baked when the balls are not firm.
Depending on the size, the amount of cookies varies. I got around 20-22 cookies.