In a large mixing bowl, combine the pumpkin puree, mascarpone cheese, granulated sugar, vanilla extract, ground cinnamon, and ground nutmeg. Mix until smooth and well combined.
In another bowl, whip the heavy cream using an electric mixer until stiff peaks form. Gently fold the whipped cream into the pumpkin mixture until smooth and creamy.
Prepare a shallow dish with the strongly brewed coffee. Dip each ladyfinger in the coffee for just a few seconds, ensuring they soak without becoming too soggy.
Begin layering your tiramisu in the baking dish. Start by placing a layer of dipped ladyfingers on the bottom of the dish.
Spread half of the pumpkin mixture over the ladyfingers in an even layer.
Add another layer of coffee-dipped ladyfingers on top of the pumpkin layer, and finish with the remaining pumpkin mixture.
Smooth the top with a rubber spatula and cover the dish with plastic wrap. Refrigerate for at least 4 hours, or overnight for best results.
Before serving, dust the top of the tiramisu with cocoa powder for a finishing touch.