In a shallow dish, combine limoncello and water. Quickly dip each ladyfinger into the mixture and arrange in a single layer in the bottom of a 9-inch springform pan.

In a large mixing bowl, beat heavy cream with an electric mixer on high speed until stiff peaks form, then set aside.

In another bowl, beat mascarpone cheese, powdered sugar, lemon juice, lemon zest, and vanilla extract until smooth and creamy.

Gently fold the whipped cream into the mascarpone mixture in three additions until fully combined and fluffy.

Spread half of the mousse mixture over the ladyfingers in the pan, smoothing the top.

Add another layer of dipped ladyfingers, then top with remaining mousse, spreading evenly.

Cover and refrigerate for at least 6 hours or overnight until fully set.

Before serving, garnish with additional lemon zest and carefully remove from springform pan.