In a small saucepan, combine water, fresh lemon juice, and sugar. Warm over medium heat, stirring, until the sugar dissolves. Do not boil hard. Remove from heat and stir in limoncello if using. Let it cool to room temperature.

In a bowl, whip cold heavy cream to soft peaks.

In another bowl, stir together mascarpone, lemon curd, zest from 2 lemons, vanilla, salt, and sugar if using. Gently fold the whipped cream into the mascarpone mixture until no streaks remain.

If using the optional egg method, whisk egg yolks with sugar over a simmering water bath until thick, then cool slightly and beat into the mascarpone.

Quickly dip each ladyfinger into the cooled lemon syrup for 1 to 2 seconds. Arrange in a snug layer in a 9-inch square pan.

Spread half the lemon mascarpone cream over the ladyfingers. Repeat with another layer of dipped ladyfingers and finish with the remaining cream.

Cover and chill for at least 6 hours or overnight for best flavor and texture.

Before serving, add lemon zest or thin lemon slices for decoration.