Meanwhile, to make the lemon filling, combine the cornflour, water, lemon juice and half the sugar in a saucepan. Use a balloon whisk to stir over medium heat for 4 minutes or until the mixture boils and thickens. Continue to cook, stirring constantly, for a further 1 minute. Remove from heat. Whisk in butter and egg yolks. Transfer to a bowl. Cover with plastic wrap and place in the fridge for 3 hours or until cooled completely.