Place the limoncello, caster sugar, lemon juice and 125ml water in a saucepan over low heat, stirring until the sugar dissolves. Remove from heat and set aside to cool.
Whisk cream and icing sugar with electric beaters until stiff peaks form. Combine the mascarpone and 200g lemon curd in a bowl, then fold in the cream mixture. Chill until ready to serve. Working with 1 glass at a time, dip 2 sponge fingers in the cooled limoncello mixture one at a time for about 10 seconds so that the sponge fingers soak up some of the liquid but do not disintegrate.
Arrange in the base of a serving glass, then top with 2 tbsp lemon cream mixture and a dollop of remaining lemon curd.
Repeat layers, finishing with cream mixture. Top with lemon zest and white chocolate. Chill in the fridge until ready to serve