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1
Make zabaglione: Set a metal bowl over a pot to create double boiler. Add enough water to reach just below bottom of bowl. Separate eggs, putting yolks in double boiler and setting whites aside. Beat yolks with 1/4 cup sugar and 1/2 cup limoncello. Heat water in double boiler until steady simmer. Whisk egg mixture until thick enough to form a ribbon when whisked, about 5 minutes. Remove from heat and cool.
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2
Meanwhile, pour remaining limoncello, 3/4 cup lemon juice, 1 cup water and 1/2 cup sugar in saucepan. Bring to a boil and cook, stirring until sugar dissolved and mixture thickens, 5 minutes. Let cool.
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3
In a large bowl, stir mascarpone to soften, then add zest and whisk until light and creamy. Using an electric mixer, whip reserved egg whites with remaining 1/4 cup sugar until mixture holds moderately firm peaks, about 3 minutes.
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4
When zabaglione is cool, use a rubber spatula to fold a third of it into mascarpone. Fold in remaining zabaglione in two or three additions, then fold in whipped egg whites in several additions. Stir gently until mixture is light and evenly blended.
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5
Fill a shallow bowl with cooled syrup to 1/4 inch depth. One at a time, roll ladyfingers in syrup and transfer to a 9 / 13 inch baking dish. Arrange moistened ladyfingers in tight rows to form a single layer of about 20 ladyfingers. Trim edges to fit.
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6
Spread half mascarpone mixture over ladyfingers. Repeat to form second layers of ladyfingers and mascarpone mixture.
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7
Cover with plastic wrap and refrigerate at least 6 hours or up to 24.
