In a large mixing bowl, combine the vegetable oil and curd until smooth.

Add the brown sugar.

Add the granulated sugar and vanilla extract.

Beat the mixture with an electric hand mixer until smooth, creamy, and fully combined.

In a separate bowl, add the all-purpose flour, cocoa powder, cornflour, baking powder, baking soda, and salt.

Whisk the dry ingredients until evenly combined.

Gradually add the dry ingredients to the wet mixture.

Fold everything together gently with a spatula until no dry streaks remain and a thick batter begins to form.

Cover the dough and refrigerate for 30–45 minutes, or until firm enough to scoop.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Scoop equal portions of dough and roll each into a smooth ball.

Roll each dough ball first in the caster sugar, then generously coat it in the icing sugar.

Arrange the coated dough balls on the prepared baking sheet, leaving about 2 inches of space between each cookie.

Bake for 10–12 minutes, or until the cookies have spread and developed their signature crinkled tops.

Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.