01 - Place egg yolks and powdered sugar in a heatproof bowl. Set over a pot of simmering water, creating a double boiler. Whisk vigorously for 2-3 minutes until the mixture thickens and turns pale yellow. Remove from heat and let cool for 5 minutes.
02 - In a chilled bowl, beat the cold heavy cream using an electric mixer or whisk until stiff peaks form. Be careful not to overbeat.
03 - Gently fold the mascarpone cheese into the cooled egg yolk mixture until smooth and fully incorporated. Fold in the whipped cream in two additions, using a spatula with light motions to maintain the airy texture.
04 - Combine cooled coffee and coffee liqueur in a shallow dish wide enough to dip ladyfingers. Stir to mix thoroughly.
05 - Quickly dip each ladyfinger into the coffee mixture for just 1-2 seconds per side—they should be moist but not soggy. Arrange half the dipped ladyfingers in a single layer in an 8-inch square dish, breaking them if needed to fit.
06 - Spread half of the mascarpone cream mixture evenly over the first layer of ladyfingers, using an offset spatula or back of a spoon to create a smooth surface.
07 - Repeat with remaining ladyfingers (dipped quickly) and top with the rest of the mascarpone cream. Smooth the top layer evenly.
08 - Cover the dish tightly with plastic wrap, ensuring it touches the surface to prevent a skin from forming. Refrigerate for at least 4 hours, 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 using a fine-mesh sieve. Add dark chocolate shavings if desired. Cut into squares and serve cold.