In a medium bowl, whisk together the egg yolks and granulated sugar until pale yellow and slightly thickened. Stir in the vanilla extract and mascarpone cheese until smooth and well combined. Set aside.

In a separate, clean bowl, beat the egg whites with an electric mixer on medium-high speed until stiff peaks form. Be careful not to overbeat.

Gently fold about one-third of the beaten egg whites into the mascarpone mixture to lighten it. Then, carefully fold in the remaining egg whites until just combined, maintaining as much airiness as possible.

In a shallow dish, combine the cooled espresso and coffee liqueur. Quickly dip each ladyfinger into the coffee mixture for just a second or two per side. Do not oversaturate.

Arrange a single layer of coffee-dipped ladyfingers in the bottom of a serving dish (an 8x8 inch or similar sized dish works well).

Spread half of the mascarpone mousse evenly over the ladyfinger layer.

Create a second layer of coffee-dipped ladyfingers over the mascarpone cream. Top with the remaining mascarpone mousse, spreading it smoothly.

Cover the dish tightly with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow the flavors to meld and the mousse to set.

Just before serving, dust the top generously with unsweetened cocoa powder using a fine-mesh sieve.