Use an electric mixer to beat the whipping cream and sugar in a large chilled bowl until peaks form. Add the mascarpone, pumpkin and spice; beat just until the filling is smooth.
Line the bottom of a 9-inch springform pan with 2 3/4-inch sides with one 3-ounce package halved ladyfingers, overlapping and crowding to fit. Sprinkle with 2 Tablespoons of rum. Spread half of the filling over the ladyfingers.
Repeat with the other package of halved ladyfingers, rum and remaining filling. Smooth. Wrap tightly with plastic, then foil.
Chill overnight. To unmold, run a knife around the inside edge of the pan. Release the pan sides; sprinkle with amaretti cookies.
If you don't own a springform pan, you can use a 9-inch square pan. You'll just have to cut servings and scoop them out of the pan- since you can't remove the sides.
Mascarpone cheese (Italian cream cheese) and amaretti cookies (Italian macaroons) are available at many supermarkets (including Trader Joes) and Italian markets.
If you don't have pumpkin pie spice on hand, it's ok to use 1/4 teaspoon each of cinnamon, cloves and nutmeg.