Place the egg yolks, whole eggs, caster sugar and lemon juice in a pot and whisk on low heat. Keep whisking until the mixture begins to thicken, about 5 minutes. Add the butter and keep stirring well with a wooden spoon. Cook until the curd coats the back of the spoon with a custard-like texture, about 30 minutes; it will thicken eventually. Take it off the heat once thick. Pour the curd through a sieve to remove any bits of scrambled egg. Store in sterilised, sealed jars; it keeps for a few months.