Cookie dough: To a large bowl or a food mixer add the oil, cocoa powder, sugar and vanilla. Mix until well combined.
Add the eggs one at a time, mixing in-between each addition until smooth and shiny.
Add the sieved flour, baking powder and salt and stir until combined.
Resting the dough: (optional) Chill the dough for 2 hours or longer, even overnight if possible. (see notes)
Preheat the oven to 170°C fan / 375°F / Gas 5.
Line a baking tray with parchment paper or a silicone mat and set it aside.
Shaping: Place the icing sugar in a bowl. Divide the cookie dough into 16 balls and gently roll in the bowl of icing sugar to coat them heavily.Coating: To ensure you get the nice finish on these cookies, coat them heavily in the icing sugar. As with resting the dough (see notes) we want to avoid the cookies having a faded appearance. Place the coated cookie balls on the prepared baking tray. (You may not be able to bake all the cookies at once as they need space around them as they bake to spread. I use a silicone mat (30x40 cm) on my baking tray and this allows me to bake a maximum of 20 cookies at one time.) I tend to bake them in 2 batches.
Baking: Bake for 7-8 minutes or until the edges are set. (See notes)
Allow the cookies to cool for 2-3 minutes on the baking sheet to continue setting, then transfer to a wire rack to cool.