Grease a 30 x 7cm loaf tin. Line base and sides with baking paper. Put chocolate and condensed milk in a heatproof bowl set over a saucepan of simmering water and stir until smooth. Remove from heat and stir in sugar, then chopped candy canes.
Spoon mixture into prepared tin and scatter over extra chopped candy cane. Chill overnight. Remove from tin, and serve cut into 24 slices.