Cookie dough: In a large bowl, mix together the oil, cocoa powder and sugar.
Add the eggs and whisk with an electric hand mixer for 1-2 minutes until the mixture has thickened slightly.
Add the sieved flour, baking powder and salt and stir until combined, but be careful not to overmix it.
Resting the dough: (optional) Chill the dough for 30 minutes or longer, even overnight if possible. (see notes)
Preheat the oven 180°C / 160°C fan / 350°F / Gas 4
Line a baking tray with parchment paper or a silicone mat and set it aside.
Shaping: Pour 35g of granulated sugar into one bowl and 35g of icing sugar into another bowl. Scoop the dough into 18-20 balls, I use a 35mm/1 tbsp cookie scoop. Roll each cookie ball lightly in granulated sugar, then generously in the icing sugar. (Rolling the cookie in granulated sugar helps to prevent the icing sugar from being absorbed into the dough, giving us that lovely crinkle effect.)
Place the coated cookie balls equally on the prepared baking trays. I use a silicone mat (30 x 40cm) on my baking tray and this allows me to bake a maximum of 12 cookies at a time. I also always bake the trays separately to avoid creating steam during the baking.
Baking: Bake for 9-10 minutes or until the edges are set. (See notes)
Allow the cookies to cool for 5-10 minutes on the baking sheet to continue setting, then transfer to a wire rack to cool.