Step 1Place the rice, 2 cups of the milk, the vanilla bean and seeds, cinnamon stick, nutmeg, cloves and orange peel in a medium heavy-bottomed saucepan. Add the sugar and salt. Cook over medium heat while stirring occasionally until the milk comes to a good simmer, about 15 minutes.
Step 2Reduce the heat to medium-low (barely a simmer) and continue to cook, stirring frequently with a wooden spoon until the rice is tender, about 1 hour. As the rice softens and the liquid reduces, the mixture will become quite thick; at this point it is important to gently stir constantly so the rice won't stick and burn onto the bottom of the pot. If the mixture is too thick, thin it out with extra milk, a couple of tablespoons at a time and up to 1 cup total.
Step 3When the rice is tender, remove from the heat. Taste for sweetness and adjust the sugar if necessary. Remove the cinnamon stick and vanilla bean pod.
Step 4Place about 1 tablespoon of chopped chocolate at the bottom of six individual heat-resistant bowls. Divide the warm rice pudding evenly among the bowls. Garnish with fresh grated orange zest if using and serve immediately.