Instructions:
- Prepare the Limoncello Syrup: Combine water and sugar in a small saucepan. Heat gently until sugar dissolves completely. Remove from heat, let cool slightly, then stir in the Limoncello and lemon zest. Set aside to cool to room temperature.
- Make the Zabaglione Base (Yolk Mixture): Beat egg yolks and sugar vigorously with an electric mixer until pale and ribbon-like (or use a double boiler method to 160°F/71°C). Gently fold in the room-temperature mascarpone until just combined and smooth—do not overmix.
- Whip the Cream: In a separate, very clean bowl, whip the heavy cream and vanilla extract until stiff peaks form.
- Fold Together: Gently fold the whipped cream into the mascarpone/yolk mixture in two additions using a rubber spatula until light and airy.
- Assembly - Layer One: Quickly dip each ladyfinger (1-2 seconds per side!) into the cooled Limoncello syrup. Arrange a single layer snugly in the bottom of a 9x13 inch dish.
- Assembly - Layer Two: Spread half of the mascarpone cream evenly over the dipped biscuits. Create a second layer of dipped ladyfingers, followed by the remaining mascarpone cream. Smooth the top gently.
- Chill: Cover the dish tightly with plastic wrap and refrigerate for a minimum of 6 hours, but ideally overnight.
- Serve: Before serving, dust generously with icing sugar through a fine sieve and finish with a sprinkle of fresh lemon zest.