Combine yolks, passion fruit juice, and sugar in a heavy-bottom saucepan then whisk to combine.
Cook over medium-high heat, stirring constantly with a wooden spoon. Make sure to scrape the sides of the pan until the mixture is thick enough to coat the back of a spoon about 8 to 10 minutes. Using a thermometer, when the mixture gets to 160 degrees remove the saucepan from heat.
Add the salt and butter, one piece at a time, stirring until smooth. Strain the curd through a fine sieve into a medium bowl.
Cover with plastic wrap, pressing it directly onto the surface of the curd to prevent a skin from forming.
Refrigerate until chilled and set, at least 1 hour to several hours.
Makes almost 2 cups