Whisk the cornstarch, cocoa and sugar in a small saucepan.
Pour the warm milk into the pot.
Heat the mixture over medium-low heat, whisking often to avoid lumps.
Bring the hot chocolate mixture almost to a light boil, then reduce the heat to low and fold in the chopped dark chocolate.
Whisk the hot chocolate until thick and creamy, adding more sugar if you like it sweeter.
Divide the Italian hot chocolate among two cups, top with whipped cream and serve.
For best results, I recommend drinking your hot chocolate straight away, as it will become denser as it cools.You can store leftovers in the fridge for up to 1 day, and reheat on the stove, adding an extra tablespoon of milk.