Bring 1" of water in a pan to boil.
Beat egg yolks, 1/4c sugar, and 1/2c liqueur in a metal or glass bowl. Place bowl over the hot water and beat another 5 minutes on highest speed until zabaglione forms a ribbon. Remove from heat.
Dump hot water and bring 1/2c sugar, 3/4c lemon juice, 1c water, and 1c liqueur to a boil. Simmer 5 minutes until syrupy. Pour into a large shallow pan to cool.
Mix mascarpone and 2 tbsp lemon zest until no longer lumpy. Fold-in cooled zabaglione in 3 batches.
Beat egg whites and 1/4c sugar to firm peaks. Fold into mascarpone mixture in 3 batches.
Dip ladyfingers into syrup, soaking both sides, then place into presentation dish to form a layer.
Cover with half the cream, and repeat with second layer of moistened ladyfingers, and remaining cream.
Cover with plastic wrap and refrigerate overnight (at least 6 hours).
Cuisine: Italian, Mediterranean, Western European Meal/Course: Dessert Occasion: Fall, Season Method: Simmering Necessary Equipment: Beaters Tips: Kid-Friendly/Family