Special equipment: a piping bag fitted with a large round tip and twelve 8-ounce parfait cups
To make the mousse, in a medium bowl, beat together the egg yolks and 1/4 cup of the sugar until light and creamy.
In a separate medium bowl, work the mascarpone with a wooden spoon until all lumps are eliminated. Add the yolk mixture, limoncello and lemon zest and stir to combine.
In a third medium bowl, combine the egg whites, salt and remaining 1/4 cup sugar and beat with an electric mixer until medium peaks form. Gently fold the egg whites into the mascarpone mixture in 3 additions. Transfer the mousse to a piping bag fitted with a large round tip.
To assemble, quickly dip 2 ladyfinger quarters into the Limoncello Simple Syrup, then put them in an 8-ounce parfait cup. Add a layer of mousse, followed by another 2 dipped ladyfinger quarters and a final layer of mousse. Continue with the remaining ladyfingers, syrup and mousse. Cover the parfait cups with plastic wrap and refrigerate for at least 3 hours.
Garnish the top of each parfait with a spoonful of berries and a sprig of basil.
In a 1-quart saucepan, combine the sugar and 1 1/2 cups water and stir to combine. Cook over medium-high heat until the mixture begins to bubble along the sides of the pot and the sugar is dissolved. Transfer to a bowl, add the limoncello and stir to combine. Set aside to cool.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.