Prepare the Coffee Soaked Sponge (Ladyfingers)

1. Preheat the Oven:
   Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
2. Whisk the Eggs and Sugar:
   In a large bowl, beat the eggs and granulated sugar together until thick and pale, about 5 minutes. The mixture should form ribbons when lifted.
3. Add the Flour and Butter:
   Sift the flour over the egg mixture and gently fold it in using a spatula. Add the melted butter and fold until just combined.
4. Pipe the Ladyfingers:
   Transfer the batter to a piping bag and pipe 3-inch long ladyfinger shapes onto the prepared baking sheet. Bake for about 10-12 minutes, or until golden and slightly firm. Remove from the oven and let them cool.
5. Soak the Ladyfingers:
   In a shallow bowl, combine the espresso and optional rum. Once the ladyfingers are cool, quickly dip each one into the coffee mixture, ensuring they are soaked but not soggy. Set aside.

Prepare the Mascarpone Mousse

6. Bloom the Gelatin:
   In a small bowl, sprinkle the gelatin over the cold water. Let it bloom for 5 minutes.
7. Heat the Espresso:
   In a small saucepan, warm the espresso (about 1/4 cup) and dissolve the bloomed gelatin into it. Stir well until completely dissolved and remove from the heat. Let it cool.
8. Whip the Cream:
   In a large mixing bowl, whip the heavy cream and powdered sugar together until soft peaks form. Set aside.
9. Mix the Mascarpone:
   In a separate bowl, whisk the softened mascarpone and vanilla extract until smooth. Slowly pour the cooled espresso-gelatin mixture into the mascarpone, stirring to combine.
10. Fold in the Whipped Cream:
    Gently fold the whipped cream into the mascarpone mixture, being careful not to deflate the mousse. Set the mousse aside.

Prepare the Coffee Syrup

11. Make the Coffee Syrup:
    In a small saucepan, combine the espresso, water, sugar, and optional rum. Heat until the sugar dissolves. Remove from heat and allow it to cool.

Assemble the Tiramisu Entremet

12. Prepare the Mold:
    Use a 7-inch (18 cm) entremet ring or a round cake ring. Line the inside of the ring with acetate strips for easy removal later.
13. Layer the Sponge:
    Start by placing the soaked ladyfingers in the bottom of the ring, arranging them to cover the base completely.
14. Add the Mascarpone Mousse:
    Spoon half of the mascarpone mousse over the ladyfingers and smooth it into an even layer.
15. Repeat the Layers:
    Add another layer of soaked ladyfingers on top of the mousse. Then, add the remaining mascarpone mousse, smoothing it evenly over the top.
16. Chill the Entremet:
    Refrigerate the entremet for at least 4 hours, or overnight, to allow the layers to set and firm up.

Prepare the Chocolate Ganache

17. Make the Ganache:
    In a small saucepan, heat the heavy cream until it begins to simmer. Pour it over the dark chocolate and let it sit for 1-2 minutes. Stir until the chocolate has melted and the ganache is smooth. Let it cool slightly.

Decorate and Serve the Entremet

18. Unmold the Entremet:
    Once the entremet has set, carefully remove the ring and acetate strips.
19. Top with Ganache:
    Pour the chocolate ganache over the top of the entremet, letting it drip down the sides. Use a spatula to smooth it out if needed.
20. Finish with Cocoa Powder and Chocolate Shavings:
    Dust the top with cocoa powder and garnish with chocolate shavings or curls. Optionally, add a few coffee beans for decoration.
21. Serve:
    Slice the entremet into individual portions and serve chilled. Enjoy!