01 - Combine water, granulated sugar, lemon juice, and lemon zest in a small saucepan. Heat over medium heat, stirring constantly until sugar completely dissolves. Remove from heat and allow syrup to cool to room temperature.
02 - Using an electric mixer, beat cold heavy cream and powdered sugar in a mixing bowl until stiff peaks form. Set aside.
03 - In a separate bowl, beat softened mascarpone cheese, vanilla extract, and lemon zest until smooth and creamy.
04 - Gently fold the whipped cream into the mascarpone mixture until fully incorporated and fluffy. Avoid overmixing to maintain light texture.
05 - Working quickly, dip each ladyfinger into cooled lemon syrup for just 1-2 seconds per side. Ladyfingers should be moistened but not soggy or falling apart.
06 - Arrange half of dipped ladyfingers in a single layer in a 9x9-inch baking dish. Spread half of mascarpone cream evenly over cookies, using spatula to smooth surface.
07 - Repeat with remaining ladyfingers and mascarpone cream, finishing with cream on top. Smooth top surface evenly.
08 - Cover dish tightly with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and texture to set properly.
09 - Before serving, decorate top with fresh lemon slices and additional lemon zest if desired. Cut into squares and serve chilled.