In saucepan combine milk, sugar, baking soda and cinnamon stick and bring to a boil over medium-high heat. Once it mixture reaches a boil, reduce heat and cook for 20 minutes at a soft oil; it will reduce about 3 cups. In a heatproof bowl whisk together eggs, then slowly pour in 1 cup of hot milk mixture, whisking continuously. Return the egg mixture to the saucepan and continue to cook over low heat, stirring often until the mixture has thickened slightly, about 7 minutes. Remove from the heat, whisk in grated chocolate and vanilla until smooth and immediately pour rompope into a heat proof bowl placed in an ice bath to chill. Discard the cinnamon stick and whisk in rum or brandy. Once mixture has cooled completely, place in fridge to chill completely. Serve chilled garnished with grated chocolate.