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.
02 - In a large bowl, whip cold heavy cream with powdered sugar using an electric mixer or hand whisk until stiff peaks form.
03 - In a separate bowl, combine mascarpone cheese, lemon zest, fresh lemon juice, and vanilla extract. Gently fold in the whipped cream until mixture is smooth and fully incorporated.
04 - Quickly dip half the ladyfingers one at a time into cooled lemon syrup. Arrange in a single layer in a 9x9-inch baking dish.
05 - Spread half the lemon mascarpone cream evenly over the ladyfingers using a spatula.
06 - Repeat dipping process with remaining ladyfingers and arrange as second layer. Top with remaining mascarpone cream, spreading evenly.
07 - Cover dish with plastic wrap or lid. Refrigerate for at least 4 hours or overnight until fully set.
08 - Before serving, garnish with fresh lemon slices and mint leaves if desired. Cut into squares and serve chilled.