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, beat cold heavy cream and powdered sugar with an electric mixer on high speed until stiff peaks form. Set aside.
03 - In a separate bowl, gently beat 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 mixture using a spatula, incorporating until fully combined and fluffy. Do not deflate the mixture.
05 - Quickly dip each ladyfinger into the cooled lemon syrup, working rapidly to ensure they are moistened but not soggy or falling apart.
06 - Arrange half of the dipped ladyfingers in a single layer in a 9x9-inch baking dish. Spread half of the mascarpone cream evenly over the ladyfingers using an offset spatula.
07 - Repeat with remaining dipped ladyfingers, then top with remaining mascarpone cream. Smooth the top surface evenly with spatula.
08 - Cover dish tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and dessert to set completely.
09 - Before serving, garnish with fresh lemon slices and additional lemon zest if desired. Cut into squares and serve chilled.