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 thoroughly 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 until ready to fold.
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, being careful not to deflate the air. Continue folding until completely smooth and uniform in texture.
05 - Quickly dip 12 ladyfinger biscuits, one at a time, into the cooled lemon syrup. Do not oversoak—just a quick dip to coat. Arrange in a single layer in the bottom of a 9x9-inch baking dish.
06 - Spread half of the lemon mascarpone cream mixture evenly over the first layer of ladyfingers, using a spatula to create a smooth, level surface.
07 - Repeat the process by dipping the remaining 12 ladyfingers in the lemon syrup and arranging them over the first cream layer. Top with the remaining mascarpone cream, spreading evenly.
08 - Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and the dessert to set completely.
09 - Just before serving, garnish with fresh lemon slices and mint sprigs if desired. Cut into squares and serve chilled.