In a medium bowl, whisk the egg yolks and sugar until smooth and creamy.

Place the bowl over a double boiler and whisk constantly for 5-7 minutes until the mixture thickens. Remove from heat and let it cool slightly.

Gently fold the mascarpone cheese into the cooled egg mixture until smooth.

In a separate bowl, whip the heavy cream and vanilla extract until stiff peaks form.

Carefully fold the whipped cream into the mascarpone mixture until fully combined.

In a shallow dish, mix the cooled espresso with the dark rum or coffee liqueur (if using).

Quickly dip each ladyfinger into the espresso mixture and place them in a single layer at the bottom of a 9x9-inch dish.

Spread half of the mascarpone mixture over the soaked ladyfingers.

Add another layer of dipped ladyfingers, followed by the remaining mascarpone mixture.

Cover the dish and refrigerate for at least 4 hours or overnight to set.

Before serving, dust the top with unsweetened cocoa powder and garnish with dark chocolate shavings if desired.