Stir together all ingredients and a pinch of salt in a heavy medium saucepan and cook, covered, over medium heat, stirring occasionally, until plums fall apart, about 25 minutes. Discard cinnamon stick and zest. Purée in batches in a blender until very smooth (use caution when blending hot liquids). Force purée through a fine-mesh sieve into a bowl, discarding solids. Cool, uncovered, then 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.

Cooks notes:

. Plum purée can be chilled up to 1 day.

. Sorbet keeps, frozen, 1 week.