Preheat the oven to 180°C (350°F). Line two baking trays with parchment paper. Place the powdered sugar in a small bowl.
Mix the wet ingredients
In a stand mixer or mixing bowl, whisk together the caster sugar and sunflower oil until glossy. Add the milk and vanilla and whisk until smooth.
Add the plain flour, cocoa powder, baking powder and salt. Mix with a spatula until a soft dough forms. It will be sticky.
Shape the cookies
Scoop tablespoons of dough (around 1½ tbsp each), roll into balls and coat generously in powdered sugar. If the dough sticks to your hands, dust your fingertips with powdered sugar.
Place the dough balls onto the trays, leaving 5 cm (2 inches) between each one.
Bake for 11 to 13 minutes until cracked on top. They should feel soft in the centre.
Let the cookies cool on the tray for 5 minutes, then move to a wire rack to cool completely.