Combine butter, sugar, lemon zest, and juice in a bowl over simmering water.
Stir until sugar dissolves, then add beaten eggs and stir until thickened.
Preheat oven to 200°C and line a 26x40cm baking tray.
Whisk eggs and sugar over simmering water until lukewarm, then whisk until thick.
Fold in sifted flour using a metal spoon.
Spread mixture onto the tray and bake for 8 to 10 minutes until firm.
Tip cake onto sugar-dusted paper and remove the parchment.
Spread warm lemon curd over the sponge.
Score a line 1cm from the edge and roll tightly using the paper.
Trim ends and dust with sugar before serving.