Cook plums and sugar in a saucepan over medium heat 5 minutes or until plums are tender and sugar is dissolved.
Process plum mixture in a food processor until smooth; strain and discard skins.
Stir together plum puree and whipping cream in container of a 2- or 4- quart electric freezer. Freeze according to manufacturer's instructions.
Pack freezer with additional ice and rock salt, and let stand 1 hour before serving. Serve with almonds, if desired.