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Once the lemon halves are soft and tender, roughly chop them and place in a pan with the lemon juice, zest, slaked cornflour and sugar. Bring to a simmer and cook gently for 10 minutes, then transfer to a blender. Blitz in the saffron and drained gelatine leaves, then add the butter a little at a time with the motor still running. Once completely smooth and fully incorporated, transfer to a bowl and cover with cling film, ensuring the film is touching the surface of the curd. Set aside in the fridge