01 - Set up a double boiler by placing a heatproof bowl over a pot of simmering water. Whisk egg yolks with powdered sugar in the bowl for 2-3 minutes until the mixture becomes thick and pale yellow. Remove from heat and allow to cool slightly.
02 - In a clean mixing bowl, beat the cold heavy whipping cream using an electric mixer or whisk until stiff peaks form. Set aside in the refrigerator until needed.
03 - Gently fold the mascarpone cheese into the cooled egg yolk mixture until smooth and fully incorporated. Carefully fold in the whipped cream until no white streaks remain. The mixture should be light, creamy, and homogeneous.
04 - In a shallow dish wide enough to dip ladyfingers, combine the cooled coffee with coffee liqueur if using. Stir well to mix.
05 - Working quickly, dip each ladyfinger into the coffee mixture for 1-2 seconds per side—just enough to moisten without becoming soggy. Arrange half of the dipped ladyfingers in a single layer in an 8-inch square dish, breaking them as needed to fit.
06 - Spread half of the mascarpone cream mixture evenly over the first layer of ladyfingers using an offset spatula or the back of a spoon.
07 - Repeat the dipping process with the remaining ladyfingers and arrange them over the cream layer. Top with the remaining mascarpone mixture, spreading smooth and level.
08 - Cover the dish tightly with plastic wrap, ensuring it does not touch the surface. Refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and texture to set properly.
09 - Just before serving, generously dust the surface with unsweetened cocoa powder using a fine-mesh sieve. Garnish with dark chocolate shavings if desired. Slice into squares and serve chilled.