In a medium size bowl mix together the cocoa powder, white sugar and oil. Add the eggs one at a time and mix well until fully incorporated. In a different bowl stir together the flour, baking powder and salt then add to the cocoa mixture and mix until it forms a soft dough. Cover with clingfilm and chill for 4 hours or overnight. When ready to bake roll one tsp of dough into small balls and coat them evenly in icing sugar, then place onto a tray lined with parchment paper. Bake in the center of the preheated oven for 10 minutes at 175 C. Keep in mind the cookies will still be soft when taking them out of the oven but they will harden as they cool.