Prepare the Caramel Layer: Melt ¼ cup sugar to make caramel. Cool slightly.
Make the Chocolate Mousse: Melt dark chocolate and set aside. Whip heavy cream until soft peaks form. Mix mascarpone with powdered sugar and vanilla, then combine with chocolate and whipped cream.
Prepare the Coffee Mixture: Cool brewed coffee and mix with dark rum or coffee liqueur if desired.
Assemble the Tiramisu: Soak ladyfingers in coffee and layer them in a dish. Spread caramel and half of the mousse above. Repeat with another layer of ladyfingers, caramel, and the remaining mousse.
Chill and Serve: Cover and refrigerate for at least 3.5 hours. Before serving, dust with cocoa and garnish with chocolate shavings.