In a mixing bowl, whisk together egg yolks, 1 whole egg, sugar, salt and yuzu juice.
Pour the mixture into a small saucepan, and heat over low heat, stirring constantly, until it reaches a temperature of 165°F/74°C and is thickened (can coat the back of a spoon).
Remove from heat and stir in the butter, until smooth and glossy.
Strain the curd through a sieve into a clean glass jar.
Cover and chill in the fridge for at least 2 hours before using.