As far as the curd is concerned, you will have better luck if you mix the yolks and sugar well together, then add the juice and the butter. Stir constantly over medium heat - we have two pans at work that *always* scorch so we always use a silicone spatula that gets into the corners. All yolks make for a firmer curd than when using a combination of whole eggs and yolks and it is definitely “eggier” (but not as eggy as a custard) but with the passionfruit, that egginess is lessened (for my taste). I like to spread a thin layer (think skim coat) on the cake layer itself before filling with passion buttercream, and we use the curd straight as a filling for our pavlova (topped with whipped cream and fruit as traditional).
Try it See what you think!