Prepare the Mascarpone Mixture: In a medium bowl, whisk together the egg yolks and sugar until smooth and creamy. Place the bowl over a double boiler, whisking constantly for 5-7 minutes until the mixture thickens. Remove from heat and allow it to cool slightly.
Incorporate the Mascarpone: Once the egg yolk mixture has cooled, gently fold in the mascarpone cheese until fully combined and smooth.
Whip the Cream: In a separate bowl, whip the heavy cream with the vanilla extract until stiff peaks form. Carefully fold the whipped cream into the mascarpone mixture until well incorporated.
Prepare the Espresso Dip: In a shallow dish, combine the cooled espresso with the rum or coffee liqueur, if using.
Assemble the Tiramisu: Quickly dip each ladyfinger into the espresso mixture and arrange them in a single layer at the bottom of a 9x9-inch dish or similar serving dish.
Layer the Mascarpone Cream: Spread half of the mascarpone mixture evenly over the ladyfingers. Repeat with a second layer of dipped ladyfingers, then top with the remaining mascarpone mixture.
Chill and Garnish: Cover and refrigerate for at least 4 hours or overnight to set. Before serving, dust the top with cocoa powder and garnish with dark chocolate shavings, if desired.