Cut the passion fruits in half and scoop out the seeds. You will need about 9 small passion fruits to get 1/2 cup of juice.
Juice the passion fruit - add the seeds to a blender. Pulse a couple of times to break down the seeds. Don't mix them for too long or the pieces will be too small to strain! Strain the seeds through a fine-mesh strainer and discard them. Measure out 1/2 cup of juice.
Make the curd using a double boiler method - prepare a medium pot and bowl (glass or non-reactive metal bowl such as stainless steel). Place the bowl over the pot with boiling water, the bowl can't touch the water surface. Add all ingredients for the curd (except for the eggs and the yolk). Whisk until the butter and sugar dissolve (over medium heat).
Reduce the heat to the minimum, add the eggs and the egg yolk, let the curd steam gently, until thickened. It will take about 10 minutes. At first, you have to mix sporadically, at the end very often. The curd should have a pudding consistency. It will keep thickening as it cools.
Enjoy!