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Heat the oven to 180C/160C fan/gas 4. Butter a 30 x 20cm deep baking dish.
Spoon the sponge batter into the dish and smooth over the top.
Mix the lemon juice with the cornflour in a heatproof bowl to make a smooth paste. Mix the remaining 130g caster sugar with 300ml boiling water in a jug, pour over the cornflour mix and whisk until smooth. Pour this over the sponge. Bake for 45-50 mins until golden and set, and the sponge springs back when touched.
While the pudding is baking, make the lemon drizzle. Mix the icing sugar with enough lemon juice (about half of it) to create a loose consistency. Drizzle over the sponge while it’s still warm and decorate with the reserved lemon zest. Serve straightaway with cream or custard.