01 - Combine water, granulated sugar, and lemon zest in a small saucepan. Bring to a simmer, stirring until sugar dissolves completely. Remove from heat, stir in limoncello, and allow to cool fully.
02 - In a large bowl, whip cold heavy cream with powdered sugar using an electric mixer or hand whisk until stiff peaks form.
03 - In a separate bowl, combine softened mascarpone cheese, lemon zest, fresh lemon juice, and vanilla extract. Mix until smooth and uniform.
04 - Gently fold the whipped cream into the mascarpone mixture using a spatula, incorporating until no streaks remain and the texture is consistently smooth.
05 - Quickly dip 12 ladyfingers individually into the cooled lemon syrup, ensuring they are moistened but not saturated. Arrange in a single layer within a 9x9-inch baking dish.
06 - Spread half of the lemon mascarpone cream evenly over the ladyfingers, using a spatula to create a uniform surface.
07 - Repeat dipping process with remaining ladyfingers and arrange over the first cream layer. Top with remaining mascarpone cream, smoothing the surface.
08 - Cover the dish with plastic wrap or lid. Refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and texture to set.
09 - Remove from refrigerator just before serving. Garnish with fresh lemon slices and mint sprigs if desired. Cut into squares and serve chilled.