Creating an incredible Christmas Tiramisu is simple when you follow these steps:

1. Prepare the Coffee Mixture: In a shallow dish, mix the brewed coffee with coffee liqueur and set aside to cool.
2. Whisk Egg Yolks: In a medium saucepan, whisk the egg yolks and granulated sugar together until smooth. Add the milk and cook over low heat, stirring constantly until the mixture thickens slightly. Remove from heat and let it cool.
3. Beat the Cream: In a large mixing bowl, whip the heavy cream until soft peaks form.
4. Combine Mixtures: Gently fold the mascarpone cheese into the cooled egg mixture until smooth. Then, fold in the whipped cream until fully incorporated.
5. Flavor the Mixture: Add vanilla extract, ground cinnamon, and nutmeg to the mascarpone mixture, folding gently to combine.
6. Soak Ladyfingers: Quickly dip each ladyfinger into the coffee mixture, ensuring not to soak them too long.
7. Layer the Dessert: In a rectangular baking dish, arrange a layer of soaked ladyfingers. Spread half of the mascarpone mixture over the ladyfingers.
8. Repeat Layers: Add another layer of soaked ladyfingers and top with the remaining mascarpone mixture.
9. Chill: Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.
10. Serve: Just before serving, dust the top with unsweetened cocoa powder and garnish with dark chocolate shavings if desired.

These steps will guide you in creating a stunning Christmas Tiramisu that is both delicious and visually appealing.