Stir together the cocoa powder, golden caster sugar and coffee in a medium bowl, then add the oil, egg and vanilla. Beat with a wooden spoon to combine, then sift the flour, baking powder, salt and cardamom directly onto the cocoa mixture. Stir for a few seconds with a wooden spoon until moistened, then add the walnuts. Stir to incorporate, then cover the bowl and refrigerate for at least 4 hours (or overnight).

When ready to bake, preheat the oven to 170°C, fan 150°C, gas 3½. Line a large baking tray with baking paper and have the icing sugar sifted and ready in a medium-size bowl.

Using a spoon, scoop out balls roughly 25g in size. Without shaping them just yet, place on a large plate – you should have about 12. Drop two or three into the bowl with the icing sugar and roll them around a little, then pick each one up and roll between your palms to form a smooth ball – the initial icing sugar coating makes it easier to roll them without them getting too sticky. Roll each ball again in the icing sugar so it is well coated this time, then place on the prepared baking tray, leaving ample space between balls as they will spread in the oven. Repeat with the remaining balls.

Bake for 10 minutes – the cookies will be soft as they come out of the oven but will firm up as they cool. Allow to cool on the baking tray for 5-10 minutes before gently transferring to a wire rack to cool completely.