Brew strong espresso and let it cool. Stir in coffee liqueur or rum (if using).

In a heatproof bowl, whisk egg yolks and sugar. Place over a simmering pot of water (double boiler) and whisk continuously for 5-7 minutes until thickened and pale.

Remove from heat and whisk in mascarpone cheese until smooth.

In a separate bowl, whip heavy cream and vanilla extract until stiff peaks form.

Gently fold whipped cream into the mascarpone mixture until smooth and fluffy.

Dip each ladyfinger quickly into the espresso mixture and layer them in a 9x9-inch dish.

Spread half of the mascarpone mixture over the ladyfingers. Repeat layers with remaining ladyfingers and mascarpone cream.

Cover and refrigerate for at least 4 hours (preferably overnight for best flavor).

Before serving, dust with cocoa powder and top with shaved dark chocolate.