Grease and line a 33cm x 23cm Swiss roll tin with buttered baking parchment.
Preheat the oven to 200°C.
Place eggs, 115g sugar, and lemon zest in a heatproof bowl over simmering water.
Whisk with an electric mixer until pale, thick, and creamy enough to leave a trail.
Remove from heat and whisk for another five minutes.
Sift in half the flour and fold in gently with a metal spoon.
Sift and gently fold in the remaining flour to maintain air in the batter.
Pour mixture into the tin and spread evenly with a spatula.
Bake for 10–12 minutes until golden-brown and firm.
Place a damp tea towel on the surface and cover with baking parchment.
Dredge parchment with sugar and flip the cake onto it, removing the lining paper.
Trim crusty edges, roll tightly from the short end with the paper inside, and cool.
Beat icing sugar and butter until creamy, then beat in lemon juice.
Gently unroll the cold cake without flattening it.
Spread lemon butter icing over the cake, leaving a 1cm border.
Spoon lemon curd evenly over the icing layer.
Roll the cake up again, place on a plate, and dust with extra sugar.