Pour a little water into a pan and bring to the boil. Stand a large heatproof bowl over the top, making sure that the base of the bowl is not touching the water. Put the sugar and lime rind in the bowl and press them against the edge of the bowl with a wooden spoon to release the oils from the rind.
Pour the lime juice into the bowl through a sieve and add the butter. Heat, stirring occasionally, until the butter has melted. Strain the eggs into the mixture and stir until well mixed. Continue heating gently for 20–30 minutes, stirring occasionally, until very thick. Take the bowl off the pan.
Halve the passion fruits, then scoop out the seeds into the lime curd using a teaspoon. Mix together gently.
Transfer to warm, dry, sterilized jars (see