In a clean mixing bowl, whisk the egg yolks and superfine sugar together for about 10 minutes until the mixture turns very pale yellow (almost white) and becomes thick and creamy. This step creates a wonderfully light and airy base for your tiramisu.
Gently stir in the vanilla extract and mascarpone cheese until just combined and smooth. Transfer this rich mixture to a separate bowl and set it aside.
Thoroughly clean your mixing bowl and whisk attachment. Add the egg whites and beat them on high speed until they form stiff, glossy peaks. This usually takes about 3 minutes and ensures a delicate, airy texture for your cream.
Carefully fold half of the mascarpone mixture into the beaten egg whites. Once mostly incorporated, add the remaining mascarpone mixture and gently fold until just combined, being careful not to deflate all the air from the egg whites. Avoid overmixing to keep the cream light.
In a shallow dish, combine the hot strong coffee with your chosen liqueur (if you're using it). Quickly dip each ladyfinger biscuit into the coffee mixture – a quick 1-2 second dip per biscuit is perfect to prevent them from becoming too soggy.
Arrange a single layer of the dipped ladyfingers evenly at the bottom of an 8-inch square serving dish.
Spread half of the airy mascarpone cream mixture over the first layer of biscuits, creating a smooth, even layer.
Repeat with another layer of coffee-dipped ladyfingers, then top with the remaining mascarpone cream, spreading it out to cover completely.
Cover the dish tightly with plastic wrap and refrigerate for at least 4-5 hours, or even better, overnight. Chilling allows the flavors to deepen and the tiramisu to set perfectly.
Just before serving, generously dust the top with unsweetened cocoa powder. Slice and serve this elegant, surprisingly light, and healthy-tasting dessert!