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 and stir in limoncello. Allow syrup to cool completely before using.
02 - In a large mixing bowl, whip cold heavy cream with powdered sugar using an electric mixer or hand whisk until stiff peaks form. Set aside.
03 - In a separate bowl, combine softened mascarpone cheese, lemon zest from 2 lemons, fresh lemon juice, and vanilla extract. Mix until smooth and well incorporated.
04 - Gently fold the whipped cream into the mascarpone mixture using a spatula. Continue folding until the mixture is uniformly smooth and no white streaks remain.
05 - Working quickly, dip 12 ladyfinger biscuits one at a time into the cooled lemon syrup. Do not oversoak—just a quick dip per side. Arrange in a single layer in the bottom of a 9x9-inch baking dish.
06 - Spread half of the lemon mascarpone cream evenly over the first layer of ladyfingers using an offset spatula or the back of a spoon.
07 - Repeat the dipping process with the remaining 12 ladyfingers and arrange them over the first cream layer. Top with the remaining mascarpone cream, spreading evenly to cover completely.
08 - Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and texture to set properly.
09 - Before serving, garnish the top with fresh lemon slices and mint sprigs if desired. Cut into squares and serve chilled.