01 - Whisk egg yolks with powdered sugar in a heatproof bowl set over a pot of simmering water, creating a double boiler. Whisk continuously for 2-3 minutes until the mixture thickens and becomes pale in color. Remove from heat and allow to cool slightly.
02 - In a separate clean bowl, beat the cold heavy cream using an electric mixer or whisk until stiff peaks form. Set aside.
03 - Gently fold the mascarpone cheese into the cooled egg yolk mixture until smooth and well incorporated. Then carefully fold in the whipped cream until fully combined and no streaks remain.
04 - In a shallow dish wide enough to dip ladyfingers, combine the cooled coffee with coffee liqueur if using. Stir gently to mix.
05 - Quickly dip each ladyfinger into the coffee mixture for just 1-2 seconds per side. They should be moistened but not soggy or falling apart. Arrange half of the dipped ladyfingers in a single layer in the bottom of an 8-inch square dish.
06 - Spread half of the mascarpone cream mixture evenly over the first layer of ladyfingers, using a spatula to create a smooth surface.
07 - Repeat the dipping process with the remaining ladyfingers and arrange them in a single layer on top of the cream. Spread the remaining mascarpone mixture evenly over the second layer of ladyfingers.
08 - Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, though overnight chilling is recommended for the best texture and flavor integration.
09 - Just before serving, remove the plastic wrap and dust the top generously with unsweetened cocoa powder using a fine-mesh sieve. Garnish with dark chocolate shavings if desired. Cut into squares and serve chilled.