1 1/2 cups chilled whipping cream
3/4 cup sugar
1 (8-ounce) container mascarpone cheese
1 (15-ounce) can pure pumpkin
3/4 teaspoon pumpkin pie spice (or 1/4 teaspoon each cinnamon, cloves, and nutmeg)
2 (3-ounce) packages halved ladyfingers
1/4 cup rum
2 ounces crushed amaretti cookies
Beat whipping cream and sugar until peaks form. Add mascarpone cheese, pumpkin, and pumpkin pie spice; beat just until filling is smooth.
Line bottom of 9-inch-diameter springform pan with 2 3/4-inch-high sides with 1 package ladyfingers, overlapping and crowding to fit. Sprinkle with 2 tablespoons rum. Spread half of filling over ladyfingers.
Repeat with second package ladyfingers, remaining 2 tablespoons rum, and remaining filling. Smooth. Wrap tightly in plastic, then foil. Chill overnight.
To unmold, run knife around inside edge of pan. Release pan sides; sprinkle with amaretti cookies.