Brew espresso and let it cool. Stir in coffee liqueur if using.

1. In a heatproof bowl over a double boiler, whisk egg yolks and sugar until pale and thickened. Remove from heat and let cool.
2. Whisk mascarpone until smooth in a separate bowl.
3. Beat heavy cream with vanilla extract until stiff peaks form.
4. Fold egg mixture into mascarpone, then gently fold in whipped cream until combined.
5. Quickly dip ladyfingers into cooled espresso (1–2 seconds each) and arrange in a single layer in an 8×8-inch dish.
6. Spread half of the mascarpone mixture over the ladyfingers.
7. Repeat with another layer of dipped ladyfingers and the remaining mascarpone cream.
8. Cover and refrigerate for at least 6 hours or overnight.
9. Before serving, dust generously with cocoa powder.