In a medium bowl combine ricotta cheese, powdered sugar, cream, vanilla, and cinnamon. Cover and chill for 4 to 24 hours.
In a medium saucepan combine plums and granulated sugar. Cook over medium heat for 15 to 20 minutes or just until plums are tender, stirring occasionally. Remove from heat; stir in lemon juice. Transfer to a large bowl. Cover and chill for 1 to 24 hours.
To serve, divide ricotta mixture among six dessert goblets or dishes. Top with plum mixture and crushed amaretti.