In a small saucepan over low heat, combine water, lemon juice, and caster sugar. Stir until the sugar dissolves, about 2 to 3 minutes. Remove from heat and cool.

In a mixing bowl, whisk mascarpone, double cream, icing sugar, and vanilla extract until thickened.

Dip half of the sponge fingers into the syrup for 1 second and arrange them in a layer in the serving dish.

Spread half of the mascarpone filling over the sponge fingers and dollop half of the lemon curd on top.

Repeat the soaking with the remaining sponge fingers and layer them atop the lemon curd, followed by the remaining cream filling and lemon curd.

Cover with plastic wrap and refrigerate for at least 4 hours or overnight.

Optionally, whip remaining double cream with icing sugar and vanilla until soft peaks form, then decorate the tiramisu with whipped cream and lemon zest before serving.