01 - Whisk egg yolks and sugar in a heatproof bowl over a pot of gently simmering water (bain-marie) until pale and slightly thickened, approximately 3–4 minutes. Remove from heat and allow to cool briefly.
02 - Gently fold mascarpone cheese and vanilla extract into the cooled yolk mixture until completely smooth and creamy, ensuring no lumps remain.
03 - In a separate chilled bowl, whip cold heavy cream until stiff peaks form. Carefully fold the whipped cream into the mascarpone mixture until just combined, being careful not to overmix and deflate the cream.
04 - Combine cooled espresso and coffee liqueur in a shallow dish wide enough for dipping ladyfingers.
05 - Quickly dip each ladyfinger into the coffee mixture for 1–2 seconds per side. Do not soak—the biscuits should remain firm in the center. Arrange a single layer of dipped ladyfingers in the bottom of a 9 x 9 inch dish.
06 - Spread half of the mascarpone cream evenly over the first layer of ladyfingers using an offset spatula. Add a second layer of coffee-dipped ladyfingers, then top with the remaining mascarpone cream. Smooth the surface evenly.
07 - Cover the dish with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and the dessert to set completely.
08 - Just before serving, dust the top generously with unsweetened cocoa powder using a fine-mesh sifter. Sprinkle with dark chocolate shavings if desired. Cut into squares and serve chilled.