In a mixer, cream the sugar and butter together. Add the flour, cornflour and cocoa powder. Cut the chocolate up into chips using a large knife, and add to the cookie mixture.
Pull the mixture out of the bowl and shape into a sausage (feeling for even distribution of chocolate chips). Cut into 16 even pieces. Roll each piece into a ball shape then place on a baking sheet (no need to grease), and press down with the prongs of a folk into a biscuit shape. Repeat with the others.