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 160 degrees F (this pasteurizes the eggs). 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 no streaks remain, creating the cream filling.
In a shallow dish, combine the cooled coffee and liqueur, if using.
Quickly dip each ladyfinger into the coffee mixture, turning once. Do not over-soak.
Arrange a single layer of dipped ladyfingers in the bottom of a 9×13 inch baking dish.
Spread half of the mascarpone cream mixture evenly over the layer of ladyfingers.
Repeat with a second layer of dipped ladyfingers, followed by the remaining mascarpone cream.
Cover the dish tightly 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 using a fine-mesh sieve.
Notes
If you prefer not to use raw or pasteurized eggs, you can substitute the egg yolk mixture with 1 cup of chilled heavy cream whipped with 1/2 cup powdered sugar until stiff peaks form, then fold into the mascarpone.
For the best flavor, use high-quality, strong coffee.
This dessert tastes better the next day, so plan to make it ahead of time.