01 - Combine water, granulated sugar, and lemon zest in a small saucepan. Bring to a simmer while stirring until sugar completely dissolves. Remove from heat, stir in limoncello, and allow to cool completely before using.
02 - In a large mixing bowl, whip the cold heavy cream together with powdered sugar until stiff peaks form. Set aside for later folding.
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 heavy 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 half of the ladyfinger biscuits one at a time into the cooled lemon syrup. Do not oversoak—just a quick dip to lightly coat each biscuit. 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 dipped ladyfingers using an offset spatula or the back of a spoon.
07 - Repeat the dipping process with the remaining ladyfingers and arrange them in a single layer over the first cream layer. Spread the remaining mascarpone cream evenly on top.
08 - Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and the tiramisu to set completely.
09 - Just before serving, decorate the top with fresh lemon slices and mint sprigs if desired. Slice into squares and serve chilled.