In a heatproof bowl set over a saucepan of simmering water (do not let the bowl touch the water), whisk the egg yolks and sugar constantly until the mixture is pale yellow, thick, and reaches 160 degrees Fahrenheit. This pasteurizes the yolks. Remove from heat and let cool slightly.
In a separate large bowl, whip the heavy cream until stiff peaks form. Set aside.
In another bowl, beat the softened mascarpone cheese until smooth. Gently 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. This is your cream layer.
In a shallow dish, combine the cooled espresso and coffee liqueur or rum, if using.
Quickly dip each ladyfinger, one at a time, into the espresso mixture. Do not soak them; a quick dip on each side is enough.
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 the 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 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
For the best flavor, use high-quality, strong espresso. Instant espresso powder mixed with hot water works well if you do not have an espresso machine.
If you omit the alcohol, add 1 teaspoon of vanilla extract to the espresso mixture for flavor.
This dessert tastes best when chilled for a full 12 hours, as it allows the ladyfingers to soften completely.