In a medium saucepan, combine fresh lemon juice, water, and granulated sugar over medium heat. Stir until sugar dissolves, about 3-5 minutes. Remove from heat, and let cool for 20-30 minutes.

In a large mixing bowl, whip heavy whipping cream with powdered sugar and vanilla extract using an electric mixer on high speed until soft peaks form, approximately 3-5 minutes.

In another bowl, beat the mascarpone cheese with lemon zest until smooth. Fold whipped cream mixture into mascarpone until fully combined.

Dip each ladyfinger into the cooled lemon syrup briefly, then layer evenly in the bottom of a dish. Spread half of the mascarpone cream mixture over the ladyfingers. Repeat with another layer, finishing with remaining cream.

Cover tightly and refrigerate for at least 4-6 hours, preferably overnight. This allows flavors to meld and dessert to firm up.

Before serving, sprinkle lemon zest or garnish with candied lemon slices. Slice into squares and serve chilled.