• 1/3 c. walnuts, toasted and coarsely chopped (see Note)
Halve plums lengthwise, and pit. Slice each half, lengthwise, into quarters. Then cut quarters in half crosswise.
Place a large heavy skillet over medium heat until bottom of pan is hot. Add plums and sprinkle with sugar, ginger and cinnamon. Stir constantly until sugar dissolves and becomes syrupy and plums are tender when pierced with a knife, about 6 minutes for medium-ripe plums. (If plums are riper, the cooking time will be shorter, only about 3 minutes.) Watch carefully, and do not overcook or plums will become mushy. Remove skillet from heat and cool plums to room temperature. Taste and if desired season with more sugar. (Plums can be prepared 3 hours ahead. Leave at cool room temperature.)
To assemble parfaits, use four sundae or wide-mouthed wine glasses. Spoon a few tablespoons of the plum mixture, including the juices, into each glass and top with a few tablespoons of the whipped cream. Sprinkle with some walnuts. Repeat this layering two more times in each glass. Garnish each parfait with a mint sprig. Serve immediately.