Prepare Espresso Mixture:
In a shallow dish, combine cooled espresso with coffee liqueur, if using. Set aside.
Make Cream Filling:
In a large bowl, whip heavy cream, sugar, and vanilla until stiff peaks form. Gently fold in mascarpone cheese until smooth and creamy.
Dip Ladyfingers:
Quickly dip each ladyfinger into the espresso mixture (do not soak; just a quick dip to avoid soggy cookies).
Assemble Tiramisu:
Arrange a layer of soaked ladyfingers in the bottom of a 9x9-inch dish. Spread half of the mascarpone cream over the layer. Repeat with another layer of soaked ladyfingers and the remaining mascarpone cream. Smooth the top.
Chill:
Cover and refrigerate for at least 4 hours, or overnight, to allow flavors to meld and the dessert to set.
Serve:
Dust with unsweetened cocoa powder and garnish with dark chocolate shavings, if desired. Slice and serve chilled.