Separate the eggs into yolks and whites.
Whisk the yolks with sugar until pale and creamy.
Add mascarpone cheese and mix until smooth.
Whip the egg whites until stiff peaks form.
Fold the whipped egg whites into the mascarpone mixture.
In a separate bowl, combine lemon juice and limoncello (if using).
Dip the ladyfingers in the lemon syrup and layer them in a dish.
Spread half of the mascarpone mixture over the ladyfingers.
Repeat the layers with the remaining ladyfingers and mascarpone mixture.
Refrigerate for at least 4 hours, preferably overnight.
Before serving, garnish with extra lemon zest and perhaps some whipped cream.
