Step 1: Prepare the Eggnog Mixture

1. In a shallow dish, mix the eggnog, spiced rum (if using), and vanilla extract. Stir well and set aside.

Step 2: Make the Mascarpone Filling

1. In a large mixing bowl, beat the mascarpone cheese until smooth.
2. In a separate bowl, whip the heavy cream and powdered sugar with an electric mixer until soft peaks form.
3. Gently fold the whipped cream into the mascarpone cheese, then add the cinnamon and nutmeg. Mix until just combined.

Step 3: Assemble the Tiramisu

1. Dip each gingerbread cookie briefly into the eggnog mixture, ensuring they are coated but not soggy.
2. Layer the soaked cookies in the bottom of a 9×13-inch dish or trifle bowl, breaking cookies as needed to fit.
3. Spread a layer of the mascarpone mixture over the cookies, smoothing it with a spatula.
4. Repeat the layers, alternating between cookies and mascarpone filling, until the dish is full or you run out of ingredients. The top layer should be mascarpone filling.

Step 4: Chill

1. Cover the dish with plastic wrap and refrigerate for at least 6 hours, or overnight for best results.

Step 5: Add the Finishing Touches

1. Just before serving, dust the top with cocoa powder or cinnamon. Garnish with crushed gingerbread cookies or candied ginger for extra flair.

Step 6: Serve

1. Slice the tiramisu into squares or scoop into individual bowls. Add optional garnishes like whipped cream swirls or grated nutmeg for an elegant presentation.