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 syrup 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, approximately 3-4 minutes. Set aside.
03 - In a separate bowl, gently beat softened mascarpone cheese, vanilla extract, and lemon zest until completely smooth and no lumps remain, about 1-2 minutes.
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 mixture is light and fluffy.
05 - Working quickly, dip each ladyfinger into cooled lemon syrup for just 1-2 seconds per side. Ladyfingers should be moistened but not saturated or soggy.
06 - Arrange half of dipped ladyfingers in a single layer at the bottom of a 9x9-inch dish. Spread half of mascarpone cream evenly over ladyfingers, using an offset spatula to smooth surface.
07 - Repeat dipping process with remaining ladyfingers and arrange as second layer. Top with remaining mascarpone cream, smoothing top surface evenly.
08 - Cover dish tightly with plastic wrap and refrigerate for minimum 4 hours, preferably overnight, to allow flavors to meld and dessert to set completely.
09 - Before serving, garnish top with fresh lemon slices and additional lemon zest if desired. Cut into squares and serve chilled.