01 - Combine water, granulated sugar, lemon juice, and zest in a small saucepan. Heat over medium heat, stirring continuously until sugar completely dissolves. Remove from heat and allow to cool thoroughly before using.
02 - Using an electric mixer on high speed, beat the cold heavy cream and powdered sugar in a mixing bowl until stiff peaks form. Set aside.
03 - In a separate bowl, gently beat the softened mascarpone cheese, vanilla extract, and lemon zest until completely smooth and no lumps remain.
04 - Gently fold the whipped cream into the mascarpone mixture using a spatula, being careful not to deflate the air. Continue folding until fully incorporated and fluffy.
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 at the bottom of a 9x9-inch dish, breaking pieces as needed to fit snugly.
07 - Spread half of the mascarpone cream evenly over the ladyfinger layer using an offset spatula, ensuring complete coverage.
08 - Repeat dipping the remaining ladyfingers and arrange them over the first cream layer. Top with the remaining mascarpone cream, smoothing the surface with a spatula.
09 - Cover the dish tightly with plastic wrap and refrigerate for a minimum of 4 hours, preferably overnight, to allow flavors to develop and the dessert to set properly.
10 - Before serving, decorate the top with fresh lemon slices and additional lemon zest if desired. Cut into squares and serve chilled.