Preheat the oven to 180'C.
In a bowl, use a spatula to mix the softened butter, sugar, salt, brown sugar and baking powder. Add the whole egg, cocoa powder and flour and mix well. Fold in the white chocolate chips and then chill in the fridge for 15-20 minutes.
Roll the dough into small balls and arrange them without plenty of space between them on a baking sheet covered with a silicone baking sheet or parchment paper.
Bake for 10-15 minutes (the dough will spread out as it bakes).
Remove the cookies from the oven when golden brown and serve whilst still warm.
«Check the cookies after 10 minutes - if they are already dry, remove from the oven so that they don't dry out too much as they cool.»