Combine 200g sugar and water in a saucepan over medium heat and bring to the boil. Stir to dissolve sugar and remove from heat. Allow syrup to cool completely. Stir in juice of 1 lemon.
Combine 200g sugar, 250ml lemon juice and lemon zest in the top of a double boiler over simmering water. Stir together until well combined. Add eggs and egg yolks. Cook and stir over simmering water until lemon mixture thickens, 15 to 20 minutes. Remove from heat and beat in butter, 1 cube at a time. Allow to cool for 10 to 20 minutes.
Beat cream in a chilled glass or metal bowl using an electric mixer until frothy. Add vanilla sugar gradually, continuing to beat until soft peaks form.
Spread 1 layer of lemon curd on the bottom of a glass dish. Dip sponge fingers in the syrup and arrange snugly in a single layer on top. Spread another layer of lemon curd on top. Spread 1 layer of whipped cream on top of the lemon curd.
Continue layering soaked sponge fingers, whipped cream and lemon curd until all ingredients are used up. Finish with a layer of lemon curd. Garnish with remaining whipped cream. For best results, refrigerate at least 3 hours before serving.