1. Whisk sugar, cinnamon, 2 teaspoons orange zest, and ¼ cup orange juice until blended; combine with plums. Let stand 15 minutes, stirring occasionally, to marinate.
2. Place water and plum mixture in medium saucepan. Simmer 18–20 minutes or until plums are fork tender. Remove from heat; let stand 7–8 minutes to cool.
3. Process in electric blender until smooth. Cover and chill (about 1 hour). Garnish soup with yogurt, if desired.

Other Preparation Method

Simmer: Combine in medium saucepan: 4 large plums (peeled, pitted, and diced), 1 ½ cups vegetable broth, 2 tablespoons each sugar and balsamic vinegar, and ½ teaspoon Chinese five-spice powder. Bring to a simmer; cook 10–12 minutes or until plums break down and soften. Process in blender until smooth. Serve as a sauce with pork or chicken.