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 heavy cream and powdered sugar with an electric mixer on medium-high speed until stiff peaks form. Set aside.
03 - In a separate bowl, gently beat 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, combining until fully incorporated and fluffy. Do not deflate the whipped cream.
05 - Working quickly, dip each ladyfinger into the cooled lemon syrup for just 1-2 seconds per side. The cookies should be moistened but not saturated or soggy.
06 - Arrange half of the dipped ladyfingers in a single layer in the bottom of a 9x9-inch dish. Spread half of the mascarpone cream evenly over the ladyfingers, smoothing with a spatula.
07 - Repeat with remaining dipped ladyfingers, creating a second layer. Top with the remaining mascarpone cream, spreading smoothly and evenly to create a flat surface.
08 - Cover the 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. Serve chilled.