Fill a saucepan with about 1 inch (2.5 cm) of water and bring it to a gentle simmer. In a large heatproof bowl, whisk together the egg yolks, sugar, and salt until smooth. Place the bowl over the saucepan, ensuring the bottom does not touch the water.

Whisk constantly for 10–12 minutes, until the mixture becomes pale, thick, and reaches 165–170°F (74–77°C). When lifted with the whisk, it should fall back in slow ribbons that briefly remain visible before disappearing.

Remove the bowl from the heat and let the mixture cool for about 10 minutes. It should still be slightly warm but no longer hot before adding the mascarpone.

Add the cold mascarpone and vanilla extract to the egg mixture. Mix on low speed only until completely smooth and creamy. Stop as soon as no streaks remain.

In a separate chilled bowl, whip the cold heavy cream until medium peaks form. The cream should hold its shape while the tips remain softly curved.

Fold one-third of the whipped cream into the mascarpone mixture to lighten it, then gently fold in the remaining whipped cream in two additions until fully incorporated. The filling should look light, airy, and completely smooth.

Combine the cooled espresso and Kahlúa (if using) in a shallow dish. Stir well.

Quickly dip each ladyfinger into the espresso mixture for about one second per side. Arrange half of them in a single layer in the bottom of the baking dish.

Spread half of the mascarpone filling evenly over the first layer using an offset spatula or the back of a spoon.

Repeat with another layer of dipped ladyfingers followed by the remaining mascarpone filling. Smooth the top into an even layer.

Cover tightly with plastic wrap and refrigerate for at least 8 hours, preferably overnight, to allow the layers to fully set and the flavors to develop.

Just before serving, sift the cocoa powder evenly over the surface. Slice with a sharp knife, wiping the blade clean between cuts for the neatest presentation.