Prepare the Limoncello Syrup

1. In a shallow dish, mix limoncello, water, and fresh lemon juice. Set aside for dipping the ladyfingers.

Make the Mascarpone Filling

1. In a heatproof bowl, whisk the egg yolks and granulated sugar. Place over a double boiler and whisk until the mixture is pale and thick (about 5 minutes). Remove from heat and let cool.

Whip the Cream

1. In a separate bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract until stiff peaks form. Gently fold in the mascarpone, ricotta (if using), lemon zest, and cooled egg mixture until smooth.

Assemble the Tiramisu

1. Quickly dip each ladyfinger into the limoncello syrup and arrange a layer in a baking dish. Spread half of the mascarpone mixture over the ladyfingers. Repeat with another layer of soaked ladyfingers and finish with the remaining mascarpone mixture.

Chill and Serve

1. Cover and refrigerate for at least 4 hours (preferably overnight) to allow flavors to meld. Before serving, garnish with white chocolate shavings or extra lemon zest.