01 - Combine lemon juice and sugar in a small saucepan. Heat over medium heat, stirring constantly until sugar completely dissolves. Remove from heat and allow to cool completely. Stir in limoncello and set aside.
02 - Beat cold heavy cream in a large bowl until soft peaks form. In a separate bowl, whisk together mascarpone, powdered sugar, vanilla extract, and lemon zest until smooth and creamy. Gently fold whipped cream into mascarpone mixture until fully incorporated and no streaks remain.
03 - Quickly dip each ladyfinger into cooled lemon syrup, ensuring brief immersion to avoid oversaturation. Arrange dipped ladyfingers in a single layer covering the bottom of a 9x9-inch dish.
04 - Spread half of mascarpone cream evenly over ladyfinger layer. Dollop half of lemon curd in spoonfuls over cream and gently swirl with spatula to create marbled pattern.
05 - Repeat process with another layer of syrup-dipped ladyfingers. Top with remaining mascarpone cream, spreading evenly. Finish with remaining lemon curd, swirling as before.
06 - Cover dish tightly with plastic wrap and refrigerate for minimum 4 hours, preferably overnight, to allow flavors to meld and texture to set properly.
07 - Just before serving, sprinkle fresh lemon zest over top. Decorate with thin lemon slices if desired. Serve cold directly from refrigerator.