Pour the milk and water in a saucepan together with the sugar, salt, zest and cinnamon powder and bring to the boil. Turn off the heat and keep warm.Toast the rice with butter in a separate sauce pan, start pouring the warm stack gradually, stirring continuously until all the liquid is absorbed and rice is soft and creamy. Remove from heat, spread on a clean tray to cool down completely. Place a spoon full of rice mixture on the palm of your hand, press in a chunk of chocolate and cover with some more rice. Press between the palms of your hand to form the arancini. Dust the rice balls with flour, dip in egg and roll in bread crumbs. Deep fry the arancini in hot frying oil until golden. Pat dry the rice balls on kitchen paper and roll in caster sugar. Serve while still hot.