01 - Whisk egg yolks with powdered sugar in a heatproof bowl set over a pot of simmering water. Cook for 2-3 minutes, whisking constantly until the mixture thickens and becomes pale yellow. Remove from heat and let cool slightly.
02 - In a chilled bowl, beat the cold heavy cream using an electric mixer or hand whisk until stiff peaks form. Set aside.
03 - Gently fold mascarpone cheese into the cooled egg yolk mixture until smooth and well incorporated. Then carefully fold in the whipped cream until no streaks remain.
04 - In a shallow dish wide enough to dip ladyfingers, combine the cooled coffee with coffee liqueur. Stir to mix thoroughly.
05 - Working quickly, dip each ladyfinger into the coffee mixture for 1-2 seconds per side. The biscuits should be moistened but not saturated or falling apart.
06 - Arrange half of the dipped ladyfingers in a single layer in the bottom of an 8-inch square dish. Spread half of the mascarpone filling evenly over the biscuits using a spatula.
07 - Repeat with remaining dipped ladyfingers to create a second layer. Top with the remaining mascarpone mixture, spreading smooth and level.
08 - Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow flavors to meld and texture to set.
09 - Just before serving, remove plastic wrap and dust the top generously with unsweetened cocoa powder. Add dark chocolate shavings if desired. Cut into squares and serve chilled.