Pour espresso (and liqueur, if using) into a shallow bowl and let cool.
In a bowl, whisk egg yolks and sugar over a double boiler until pale and thick (about 5 minutes), then cool slightly.
Mix mascarpone and vanilla into the yolk mixture until smooth.
Whip cold heavy cream to stiff peaks in a separate bowl, then gently fold into mascarpone mixture.
Quickly dip each ladyfinger into coffee mixture (don’t soak) and lay half in pan.
Spread half of the cream mixture over the ladyfingers.
Repeat with remaining dipped ladyfingers and cream.
Cover and refrigerate for at least 2 hours, preferably overnight, to set.
Before serving, dust the top liberally with cocoa powder.
Notes
Be cautious not to over‑soak the ladyfingers to avoid a soggy dessert. For food safety, pasteurized eggs or zabaglione method should be used when serving raw yolks.