Whisk together the cocoa powder, caster or granulated sugar, oil, milk and vanilla extract in a mixing bowl until smooth.
65 g (¾ cup) cocoa powder, 150 g (¾ cup) caster or granulated sugar, 55 g (¼ cup) neutral oil (I use olive), 80 g (⅓ cup) unsweetened non-dairy milk (I use soy), 1 teaspoon vanilla extract
Mix in the baking powder, salt and espresso powder, followed by the flour. Stir until no dry lumps remain, the dough should resemble a thick brownie batter.
1 teaspoon baking powder, 1 teaspoon instant espresso powder, ¼ teaspoon salt, 140 g (1 cup + 2 ½ Tablespoons) plain (all-purpose) flour
Cover the bowl and chill the dough in the fridge for at least 1 hour, preferably 4.
Preheat the oven to 190℃/170℃ fan/375℉/gas mark 5. Line two baking sheets with baking parchment. Place the caster/graulated sugar in one small bowl, and the icing/powdered sugar in another.
50 g (¼ cup) caster or granulated sugar, 50 g (½ cup) icing (powdered) sugar
Divide the chilled dough into 16 portions, about a heaped Tablespoon each, and roll them into balls.
Roll the balls first in the caster/granulated sugar, giving them a light coating, and then in the icing/powdered sugar, making sure that they have a thick, even coating.
Place the cookies spaced apart on the lined baking sheets and bake for 12-15 minutes until firm around the edges but still slightly wet looking in the centre.
If you prefer flatter cookies, bash the baking sheets on the counter as you take them out of the oven, this will deflate the cookies slightly.
Allow the cookies to cool on the baking sheets for 5-10 minutes, then gently transfer them to a wire rack to cool completely.