Prepare the coffee mixture by brewing strong coffee and allowing it to cool. Mix with coffee liqueur if using.
Quickly dip ladyfingers into the coffee mixture and lay them side by side on a serving platter.
In a mixing bowl, whip heavy cream until soft peaks form. Gradually add powdered sugar and vanilla extract, then fold in mascarpone cheese until smooth.
Spread half of the mascarpone mixture over the layer of soaked ladyfingers, then add another layer of soaked ladyfingers on top.
Spread the remaining mascarpone mixture over the second layer of ladyfingers and shape into a log.
Cover with plastic wrap and refrigerate for at least 4 hours or overnight.
Before serving, dust with cocoa powder and sprinkle with dark chocolate shavings.
Notes
For best results, refrigerate overnight to allow flavors to meld.