Make the Pumpkin Mascarpone Layer: In a large bowl, beat mascarpone, pumpkin puree, brown sugar, vanilla, and pumpkin pie spice until smooth and fluffy.
Whip the Cream: In a separate chilled bowl, beat heavy cream and powdered sugar until soft peaks form.
Fold Together: Gently fold whipped cream into the pumpkin mascarpone mixture until combined.
Layer the Tiramisu: Dip ladyfingers quickly into coffee (with or without liqueur). Arrange a layer in the bottom of a 9x9" dish.
Spread half of the pumpkin mixture over the ladyfingers. Repeat with another layer of dipped ladyfingers and the rest of the pumpkin mixture.
Chill: Cover and refrigerate at least 6 hours (or overnight) to let flavors meld and layers set.
Serve: Before serving, dust with ground cinnamon. Slice and swoon.