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 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, fresh lemon juice, and vanilla extract. Mix until smooth and creamy.
04 - Gently fold the whipped cream into the mascarpone mixture using a spatula. Continue folding until fully incorporated and no streaks remain. The mixture should be light, fluffy, and smooth.
05 - Quickly dip 12 ladyfingers one at a time into the cooled lemon syrup—do not soak, just a brief dip to absorb flavor. Arrange in a single layer in the bottom of a 9x9-inch dish.
06 - Spread half of the lemon mascarpone cream evenly over the ladyfinger layer using a spatula, ensuring complete coverage.
07 - Repeat dipping process with remaining 12 ladyfingers and arrange as second layer. Top with remaining mascarpone cream, spreading smoothly to edges.
08 - Cover dish tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight. This allows flavors to meld and texture to set properly.
09 - Before serving, garnish with fresh lemon slices and mint leaves if desired. Cut into squares and serve chilled.