Place the lemon zest and juice in a pan over a medium heat and bring to the boil

Meanwhile, mix the cornflour into the water to form a smooth paste. In a separate bowl, whisk together the sugar and egg yolks then stir in the cornflour mixture

Pour this into the boiling lemon juice, whisking continuously until the mixture is smooth and has thickened

Remove the pan from the heat and beat in the butter a little at a time until smooth and glossy

Transfer to a clean jar and leave to cool. The curd will keep for a week in the fridge