Place the gelatin and water in a small bowl, let soak for 5 minutes and then drain and set aside. In a large bowl, whisk the yolks, cornstarch, salt and sugar. In a large saucepan, heat the milk just until steaming. Whisk 1 cup of the hot milk into the yolk mixture. Pour the whole mixture into the milk in the saucepan and cook over moderate heat. Whisk constantly, until the mixture is boiling and thick, about 5 minutes. Whisk in the pumpkin butter and cook, whisking for 1 minute. Off the heat, whisk in the gelatin, vanilla and cinnamon. Whisk in the mascarpone. In a small microwave-safe bowl, microwave the Calvados for 10 seconds. Arrange one-third of the gingersnaps in a 9 x13 x 2 1/2-inch baking dish. Brush top with one-third of the pumpkin custard. Repeat the layering twice more with the remaining gingersnaps, Calvados and custard. Press a sheet of plastic wrap directly on the surface of the tiramisù. Freeze overnight or at least 5 hours. Let the tiramisù stand at room temperature for 6 hours, until thawed. Sprinkle with the remaining gingersnaps. Serve. MAKE AHEAD: The pumpkin tiramisù can be frozen for up to 1 week. Thaw at room temperature overnight in the refrigerator.