In a small bowl, bloom the gelatin in water.**
In a medium sized sauce pot, bring the passion fruit juice, sugar and zest to a simmer over medium heat. Cook until the sugar is dissolved. Turn off heat.
In a medium bowl, whisk together the whole eggs, yolks, and salt.
Whisk a 1/4 cup of the hot juice into the egg mixture. Repeat with more juice until the eggs are warm. Once the eggs are warm, whisk them into the pot of remaining juice.
Return the pot to a low heat. Using a rubber spatula, stir the mixture constantly, making sure to clean the sides of the pot as you go, until the passion fruit curd begins to thicken, about 5 minutes; it will be the consistency of smooth pudding and will reach 165°F on an instant read thermometer.
Remove the pot from the heat and stir in the bloomed gelatin and butter until melted.
Strain it through a fine-mesh sieve and immediately spread evenly over the baked crust. Refrigerate until set, about 3 hours. (Can be made several hours before cutting and serving.)
Cut into bars using a sharp knife dipped into hot water. Top with a dusting of confectioners' sugar or the Meringue Topping below. If topping with meringue, set the bars on a baking sheet that is NOT lined with parchment, so you can use the blow torch to finish.