place 1 cup of the milk into a saucepan. Add lemon peel and cinnamon sticks. Bring milk to a simmer over medium heat.
Meanwhile, in a small bowl, whisk together the cornstarch and sugar. Whisk remaining milk into the cornstarch mixture. When the milk in the pan begins to simmer, pour the cornstarch mixture into the saucepan in a thin steady stream. Whisk vigorously and increase heat just a bit to bring the mixture to a gentle boil. Allow the mixture to boil for about 20 seconds while continuing to whisk, then remove from heat. Remove the lemon peel and cinnamon sticks. Stir in the pinch of cinnamon and, if using, the vanilla.
Pour into a dish or mold, and refrigerate for at least 6 hours, or overnight. Garnish with shaved chocolate just before serving.