Place butter, sugar, juices, zest and passionfruit in a stainless steel bowl over a double boiler of simmering water on medium-low heat and stir until melted.
In a separate bowl beat 2 eggs. Whisk into the stainless steel bowl the beaten eggs and yolk and stir continuously over a gentle heat until smooth and thick.
Pour into a sterilised jar and store in the fridge (curd can be sieved to remove some or all the seeds).
Tip: Best served chilled in the fridge for at least one hour.