Mix the cocoa, 200g caster sugar and oil together with most of the orange zest. Whisk in the eggs one at a time, 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 you form a soft dough. If the dough feels very soft, transfer it to the fridge and chill for 1 hr.
Heat the oven to 190C/170C fan/gas 5 and line a baking tray with baking parchment. Tip the remaining caster sugar into a small bowl. Add a drop of food colouring and stir it through the sugar until it’s all coloured – add more if you want a stronger colour. Stir in the icing sugar until all of the sugar is coloured, then stir in the remaining orange zest.
Form a heaped teaspoon of the dough into a ball, then roll in the sugar to coat, you might have to press the sugar on, particularly if you’ve chilled the dough. If the sugar is not sticking well, roll the balls of dough between your hands until the mixture warms up a little. Repeat with the remaining dough, then put, evenly spaced, on the baking tray.
Bake in the centre of the oven for 10 mins – they will crinkle as they cook. The biscuits will firm up as they cool so don’t overcook them. Transfer to a wire rack and leave to cool. Will keep for four days in a biscuit tin.