In a large saucepan, combine the milk, cinnamon stick and orange zest and cook over moderately high heat until bubbles appear around the edge. Remove from the heat, cover and let steep for at least 30 minutes and up to 2 hours.
Remove and discard the cinnamon stick and orange zest. Whisk the cocoa and sugar into the milk until thoroughly blended. Add the cream and bittersweet chocolate and cook over moderate heat, whisking constantly, until the cocoa is smooth. Serve piping hot.