Step 1: In a small saucepan, combine the water, granulated sugar, and lemon juice. Heat over medium heat, stirring until the sugar is completely dissolved. Remove from heat and stir in 1/4 cup of the Limoncello. Let the syrup cool completely.
Step 2: In a large mixing bowl, whisk the mascarpone cheese with the lemon zest and the remaining 1/4 cup of Limoncello until smooth and creamy.
Step 3: In a separate chilled bowl, beat the heavy whipping cream and powdered sugar together until stiff peaks form.
Step 4: Gently fold the whipped cream into the mascarpone mixture in three batches, being careful not to deflate the air.
Step 5: Quickly dip each ladyfinger into the cooled lemon syrup (about 1 second per side) and arrange them in a single layer at the bottom of an 8×8 or 9×9 inch dish.
Step 6: Spread half of the mascarpone cream over the ladyfinger layer, smoothing it out with a spatula.
Step 7: Repeat with another layer of dipped ladyfingers and finish with the remaining cream.
Step 8: Cover and refrigerate for at least 6 hours, preferably overnight, to allow the ladyfingers to soften and flavors to develop.
Step 9: Garnish with additional lemon zest or fresh berries before serving.