Step 1: Prepare the Limoncello Syrup

1. In a shallow dish, combine limoncello, water, and the zest of two lemons. Mix well.
2. Taste the mixture and adjust the sweetness with a tablespoon of sugar if needed. Set aside.

Step 2: Make the Mascarpone Filling

1. In a large mixing bowl, beat the mascarpone cheese with a hand mixer until smooth and creamy.
2. In a separate bowl, whip the heavy cream with powdered sugar and vanilla extract until stiff peaks form.
3. Gently fold the whipped cream into the mascarpone cheese until well combined. Be careful not to overmix to maintain the fluffy texture.

Step 3: Assemble the Tiramisu

1. Dip each ladyfinger into the limoncello syrup briefly, ensuring they are moistened but not soggy.
2. Arrange a layer of soaked ladyfingers at the bottom of a 9×13-inch dish or trifle bowl.
3. Spread half of the mascarpone filling evenly over the ladyfingers.
4. Repeat with another layer of soaked ladyfingers and the remaining mascarpone filling.
5. Smooth the top with an offset spatula.

Step 4: Chill and Garnish

1. Cover the dish with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow the flavors to meld.
2. Just before serving, garnish with lemon zest, white chocolate shavings, and fresh mint leaves if desired.