In a medium saucepan over medium heat, combine 1/2 cup of fresh lemon juice, 1/2 cup of water, and 1/4 cup of granulated sugar. Stir until sugar dissolves and mixture simmers gently for about 5 minutes. Remove from heat and let cool.

In a large mixing bowl, whip 1 cup of cold heavy whipping cream with 1/4 cup of powdered sugar and 1 teaspoon of pure vanilla extract until soft peaks form, about 3-4 minutes.

In a separate bowl, beat 1 cup of mascarpone cheese with the zest of 1 lemon until smooth. Combine both mixtures and fold gently to maintain airiness.

Dip ladyfinger cookies quickly into the cooled lemon syrup for 2-3 seconds, then layer them in a 9x9-inch dish. Spread half of the mascarpone mixture over the ladyfingers.

Repeat dipping and layering with remaining ladyfingers and mascarpone mixture, ensuring that layers align neatly.

Cover tightly with plastic wrap and refrigerate for at least 4-6 hours, or overnight for best flavor.

Before serving, garnish with extra lemon zest or candied lemon slices.