Slice each passion fruit in half and scoop the pulp into a blender. On low speed, pulse the blender to separate the seeds from their gelatinous membrane, but not enough to puree the seeds. Rest a fine mesh sieve over a small bowl and pour the passion fruit into the sieve. Use the back of a spoon to press out the juice, stirring and pressing several times, until you're just left with pulp and seeds in the sieve. Discard pulp and seeds. You should have about 1/2 cup of passion fruit juice.
In a small saucepan, combine the egg yolks, passion fruit juice and sugar. Stir constantly over medium heat until the sugar dissolves and the sauce thickens enough to coat the back of a spoon.
Remove from heat. Add salt and butter, one chunk at a time, stirring after each addition until the butter melts.
Place a clean sieve or fine mesh strainer over a small bowl and pour the curd into the strainer. Stir the curd, pushing gently through the sieve (you're removing any egg solids from the curd by straining a second time -- and ensuring a silky smooth texture). Transfer curd to a serving bowl and press a piece of plastic wrap to the top of the curd to avoid a skin. Refrigerate until chilled. Can be made one day ahead.