In a bowl, combine the mascarpone and powdered sugar. Mix with an electric mixer for a couple of minutes until smooth.
Gradually add the cold heavy cream, then the lemon zest, lemon juice, and 2 tsp (10ml) limoncello (or lemon juice for a non-alcoholic version). Continue mixing until you get a thick, creamy consistency.
In a small saucepan, combine the water, granulated sugar, and lemon zest. Stir and bring to a boil.
Turn off the heat and let the syrup cool completely. Once cooled, transfer to a bowl and add 1 tbsp + 1 tsp (20ml) limoncello (or lemon juice).
Dip the ladyfingers quickly into the syrup (about 2 seconds each), then layer them in a tiramisù dish.
Spread a layer of the lemon mascarpone cream over the soaked ladyfingers. Continue layering until ingredients are used up.
Decorate with lemon slices and refrigerate for at least a couple of hours (ideally overnight). Enjoy!
