Add 1.25 cups of egg nog and rum into a bowl. Whisk to combine. Set aside.

Place 28 ladyfingers into a large pan so they are lying flat. Pour the eggnog mix on top. Set aside. They should soak for at least 10 minutes. About the time it will take you to prepare the mascarpone filling.

Add the heavy cream, remaining eggnog, powdered sugar, vanilla, cinnamon, and nutmeg to a large mixing bowl. Using a hand mixer, whisk until soft peaks form, about 3-4 minutes. Add the mascarpone and mix until stiff peaks form about 2 minutes. Set aside.

To assemble, spread ⅓ of the whipped cream mascarpone mixture into the bottom of a 9x9-inch pan. Next, lay down 12-14 ladyfingers so they are lying flat and not overlapping. Spread another ⅓ of the cream mixture over the ladyfingers. Continue with the next and final layer.

Cover with plastic wrap and refrigerate for at least 5 hours or overnight.

Before serving dust the top with cinnamon and nutmeg.