01 - Combine water, granulated sugar, lemon juice, and zest in a small saucepan. Heat over medium heat, stirring constantly until sugar completely dissolves. Remove from heat and allow to cool completely before using.
02 - In a chilled mixing bowl, whip the cold heavy cream and powdered sugar using an electric mixer on high speed until stiff peaks form, approximately 3-4 minutes.
03 - In a separate bowl, gently beat the softened mascarpone cheese, vanilla extract, and lemon zest until smooth and creamy, taking care not to overmix.
04 - Gently fold the whipped cream into the mascarpone base using a spatula, incorporating until fully combined and fluffy. Set aside.
05 - Quickly dip each ladyfinger into the cooled lemon syrup, working one at a time. Ensure each cookie is moistened but not saturated or soggy. This should take 1-2 seconds per ladyfinger.
06 - Arrange half of the dipped ladyfingers in a single, tight layer in the bottom of a 9x9-inch baking dish, breaking pieces as needed to fill gaps.
07 - Spread half of the mascarpone cream mixture evenly over the ladyfinger layer using an offset spatula, ensuring complete coverage.
08 - Create a second layer with the remaining dipped ladyfingers, then top with the remaining mascarpone cream. Smooth the top surface with a spatula for an even finish.
09 - Cover the dish tightly with plastic wrap and refrigerate for a minimum of 4 hours, preferably overnight, to allow flavors to meld and the dessert to set properly.
10 - Just before serving, decorate the top with fresh lemon slices and additional lemon zest if desired. Cut into squares and serve chilled.