WHISK the eggs, sugar, cornflour and lemon juice together in a bowl until smooth.
POUR the mixture into a saucepan and place over a low heat.
COOK gently, whisking continuously for a few minutes until the mixture thickens into a silky curd. Do not allow it to boil.
REMOVE from the heat and immediately whisk in the butter, stirring continuously until fully melted and glossy.
STRAIN the lemon curd through a fine sieve into a bowl or clean glass jar for an extra smooth finish.
REFRIGERATE for at least 3 hours or until completely cold and set.