01 - Combine water, granulated sugar, and lemon zest in a small saucepan. Bring to a simmer, stirring until sugar completely dissolves. Remove from heat, stir in limoncello, and allow to cool completely before using.
02 - In a large 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, lemon zest, 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, being careful not to deflate the air. Continue folding until fully combined and no streaks remain.
05 - Quickly dip half of the ladyfingers one at a time into the cooled lemon syrup, ensuring they absorb liquid but don't become soggy. Arrange in a single layer in a 9x9-inch baking dish.
06 - Spread half of the lemon mascarpone cream evenly over the first layer of ladyfingers using a spatula, reaching the corners of the dish.
07 - Repeat dipping process with remaining ladyfingers and arrange as second layer. Top with remaining mascarpone cream, spreading evenly to create a smooth surface.
08 - Cover the dish 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, decorate with fresh lemon slices and mint sprigs if desired. Cut into squares and serve chilled.