01 - Combine water, granulated sugar, and lemon zest in a small saucepan. Bring to a simmer, stirring constantly until sugar completely dissolves. Remove from heat, stir in limoncello, and allow mixture to cool completely before using.
02 - In a large mixing bowl, beat cold heavy cream with powdered sugar using an electric mixer or hand whisk until stiff peaks form. Set aside in refrigerator.
03 - In a separate bowl, combine softened mascarpone cheese, lemon zest, fresh lemon juice, and vanilla extract. Mix until smooth and well incorporated.
04 - Gently fold the whipped heavy cream into the mascarpone mixture using a spatula, taking care not to deflate the air. Continue folding until completely smooth and uniform.
05 - Working quickly, dip half of the ladyfingers one at a time into the cooled lemon syrup. Do not soak—just a quick dip to moisten. Arrange in a single layer in the bottom of a 9x9-inch dish.
06 - Spread half of the lemon mascarpone cream evenly over the first layer of ladyfingers, using a spatula to create a smooth surface.
07 - Repeat the dipping process with remaining ladyfingers and arrange them over the first cream layer. Top with remaining mascarpone cream, spreading evenly to cover completely.
08 - Cover the dish tightly with plastic wrap or foil. Refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and dessert to set properly.
09 - Before serving, remove from refrigerator and garnish with fresh lemon slices and mint sprigs if desired. Slice and serve cold.