Combine the sugar, cornflour, cocoa powder and milk in a small saucepan over medium heat. Bring to a simmer, and stir until thick, about 2 minutes. Remove from heat; stir in vanilla and butter. Transfer to a heatproof bowl, and refrigerate until cool, approximately 20 minutes.
Stir the chocolate chips into the cooled chocolate mixture. Pour into moulds and freeze until firm, about 4 hours.