01 - Combine water, sugar, and lemon zest in a small saucepan. Bring to a simmer over medium heat, stirring constantly until sugar completely dissolves. Remove from heat, stir in fresh lemon juice, and allow to cool completely before using.
02 - Beat heavy cream with powdered sugar and vanilla extract in a large bowl until soft peaks form. In a separate bowl, gently whisk mascarpone with lemon zest until smooth and creamy. Fold whipped cream into mascarpone mixture until fully incorporated and fluffy.
03 - Quickly dip each ladyfinger into cooled lemon syrup, ensuring just a brief dip to absorb liquid without becoming soggy. Arrange dipped ladyfingers in a single layer at the bottom of a 9x9-inch baking dish.
04 - Spread half of the mascarpone mixture evenly over the first layer of ladyfingers, using a spatula to create a smooth, uniform surface.
05 - Create another layer of dipped ladyfingers, then top with remaining mascarpone mixture. Smooth the top surface evenly with a spatula.
06 - Cover dish tightly with plastic wrap or aluminum foil. Refrigerate for at least 4 hours, preferably overnight, to allow flavors to meld and texture to set properly.
07 - Just before serving, garnish top with thin lemon slices and fresh mint leaves if desired. Cut into squares and serve chilled.