This will be your cupcake filling, but it can also be spread on toast, waffles, ice cream or other items. This filling is sweet and a little goes a long way.
Pour passion fruit juice and sugar into medium-sized pan on stove top. Heat just until sugar is dissolved, stirring the whole time. Remove from heat.
In medium-sized bowl, whisk egg yolks and lemon juice until combined.
Pour passion fruit and sugar mixture into egg mixture, just a little bit at a time. (Pouring in a very thin stream, whisking constantly and stopping every few seconds is best, as you don't want the eggs to cook. A little of the warm mixture at a time allows the eggs to slowly adapt to the temperature, aka tempering, without cooking them.)
After entire fruit and sugar mixture is combined into egg and lemon juice mixture, pour it all back into the medium-sized stove pan.
Turn heat back on low-medium, and whisk constantly until temperature of mixture gets to 160 degrees AND thickens up. (It took my mix 18 minutes to reach temperature and thicken, but this is stove and pan-dependent.) Remove from heat immediately and keep stirring until you pour the mixture into a bowl. Do not leave the mixture sitting -- it will continue to cook, and possibly will burn and/or separate or curdle.
Let cool entirely before you fill the cupcake wells with curd.