Mix the cocoa, caster sugar and oil together. Add the eggs one at a time, whisking until fully combined.
Stir the flour, baking powder and a pinch of salt together in a separate bowl, then add to the cocoa mixture and mix until a soft dough forms. If it feels soft, transfer to the fridge and chill for 1 hr. Heat the oven to 190C/170C fan/gas 5. Tip the icing sugar into a shallow dish. Form a heaped teaspoon of the dough into a ball, then roll in the sugar to coat. Repeat with the remaining dough, then put, evenly spaced, on a baking tray lined with baking parchment.
Bake in the centre of the oven for 10 mins – they will firm up as they cool. Transfer to a wire rack and leave to cool. Will keep for four days in a biscuit tin.