Split vanilla beans in half lengthwise. With a sharp knife, scrape the beans to remove the seeds. Set seeds aside; discard the beans.
In a large saucepan, heat milk and 3/4 cup cream over medium heat just until mixture comes to a simmer. Chop 15 ounces chocolate. In a large heatproof bowl, combine the chopped chocolate, Nutella, sugars, salt and vanilla seeds. Pour warm milk mixture over chocolate mixture. Let stand for 1 minute.
Meanwhile, whip remaining cream and shave remaining chocolate. Whisk chocolate mixture until smooth; stir in liqueur. Serve with whipped cream, shaved chocolate and chopped hazelnuts. Garnish with a cinnamon stick if desired.