Grease a square cake tin. Chuck condensed milk, butter, syrup and sugar in a saucepan (a non stick one makes life much more pleasant...) and combine over low heat, stirring constantly until mixture is glossy and sugar has dissolved. Keep stirring and bring to the boil until it thickens and starts to pull away from the sides. Remove from heat, stir in chocolate and additives, pour into tin and bung into the fridge. Cut with a hot knife.