Make a spice bag with the cinnamon stick, cloves, star anise, and peppercorns. (Otherwise you'll need to count and fish them all out before you puree the mixture.)
Add prepared plums, red wine, sugar, water, spice bag, lemon juice and zest, and a pinch of salt to a heavy medium saucepan and cook, covered, over medium heat, stirring occasionally, until plums fall apart, about 25 minutes.
Discard whole spices and zest. Pour mixture into a deep mixing bowl, and allow it to cool. Purée using an immersion blender* until very smooth. Force purée through a fine-mesh sieve into a bowl, discarding any solids.
Chill, covered, until cold, at least 2 hours.
Freeze purée in ice cream maker, then transfer sorbet to an airtight container and put in freezer to harden, at least 1 hour.
Using an ice cream scoop, place 2 rounded scoops into each glass. Garnish with mint sprig and a square of dark chocolate if desired.