1. Pour espresso into a shallow dish and stir in coffee liqueur if using. 2. Whisk egg yolks and sugar in a heatproof bowl over barely simmering water for 6 to 8 minutes until pale and thick. Cool for 10 minutes. 3. Beat mascarpone briefly until smooth and fold in the yolk mixture. 4. Whip cream and vanilla to medium peaks. Fold into the mascarpone mixture in three additions. 5. Dip each ladyfinger quickly into espresso, about 1 second per side. 6. Arrange one layer in a 9 inch square dish and spread with half the mascarpone cream. 7. Add another layer of dipped ladyfingers and the remaining cream. 8. Cover and refrigerate for at least 6 hours, preferably overnight. 9. Dust with cocoa powder just before serving and add grated chocolate if desired.