Preheat oven to 180°C bake.
Prepare your baking trays with baking paper.
Cream the sugar, butter and vanilla until light and fluffy.
Sift in the flour, cocoa and cornflour. Blend until mostly combined, then add the cornflakes and mix well.
Roll tablespoons or use a medium sized cookie scoop, of mixture into balls and place on prepared trays. Press down and lightly flatten with clean wet hand.
Bake for 15 minutes, until biscuits are just firm to the touch.
Leave on trays for 10 minutes, then transfer to a wire rack to cool completely.