Instructions

Put the lemon juice and zest into a pan and bring to the boil.

Mix the cornflour (cornstarch) 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.