In a heatproof bowl set over a saucepan of simmering water (double boiler), whisk the egg yolks and sugar constantly until the mixture is pale yellow and thick, reaching 160F. Remove from heat and let cool slightly.
In a separate large bowl, whip the cold heavy cream until stiff peaks form. Set aside.
In another bowl, gently whisk the softened mascarpone cheese until smooth. Fold the cooled egg yolk mixture into the mascarpone until just combined.
Gently fold the whipped cream into the mascarpone mixture until fully incorporated, creating the cream.
Combine the cooled coffee and liqueur (if using) in a shallow dish.
Quickly dip each ladyfinger, one at a time, into the coffee mixture. Do not soak them too long or they will become soggy.
Arrange a single layer of dipped ladyfingers in the bottom of an 8×8 inch baking dish.
Spread half of the mascarpone cream mixture evenly over the ladyfingers.
Repeat the layering process: dip the remaining ladyfingers and arrange them over the cream layer.
Spread the remaining mascarpone cream over the second layer of ladyfingers.
Cover the dish with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow the dessert to set.
Before serving, dust the top generously with unsweetened cocoa powder.
Notes
For a recipe without raw eggs, you can substitute the egg yolks with 1 cup of heavy cream whipped with 1/4 cup of powdered sugar, then fold into the mascarpone.
Use high-quality, strong coffee for the best flavor.
This dessert tastes better when made a day ahead, as the flavors meld together.