Prepare the Chocolate Mousse:

1. Melt the Chocolate: In a heatproof bowl, melt the semisweet chocolate either in the microwave in 30-second intervals or over a double boiler. Set aside to cool slightly.
2. Whip the Cream: In a large mixing bowl, whip the heavy cream with an electric mixer on medium-high speed until soft peaks form.
3. Mix Mascarpone and Sugar: In a separate bowl, whisk together the mascarpone cheese, powdered sugar, and vanilla extract until smooth and creamy.
4. Combine the Chocolate and Cream: Gently fold the melted chocolate into the mascarpone mixture, followed by the whipped cream, until fully combined. Set aside.

Prepare the Coffee Mixture:

5. Brew the Coffee: Brew a strong cup of coffee or espresso and let it cool to room temperature.
6. Add Liquor (Optional): If using, add the dark rum or coffee liqueur to the cooled coffee, stirring to combine.

Assemble the Tiramisu:

7. Soak the Ladyfingers: Quickly dip each ladyfinger into the coffee mixture, making sure not to soak them too long as they can become soggy. Arrange the dipped ladyfingers in the bottom of a 9×9-inch or similar-sized serving dish.
8. Layer the Mousse: Spread half of the chocolate mousse over the soaked ladyfingers, smoothing it into an even layer.
9. Repeat the Layers: Add another layer of dipped ladyfingers, followed by the remaining chocolate mousse. Smooth the top layer of mousse evenly.

Chill and Serve:

10. Chill the Tiramisu: Cover the dish with plastic wrap and refrigerate for at least 3.5 hours, or overnight, to allow the flavors to meld and the dessert to firm up.
11. Garnish and Serve: Before serving, dust the top of the tiramisu with cocoa powder and garnish with chocolate shavings or curls for an extra touch of elegance.