Preheat the oven to 180°C (350°F). Line a 20cm (8-inch) square baking pan with baking paper.
To make the crust: In a large bowl combine all the crust ingredients until well combined (start with 2tbs or 60mL of sweetener). The mixture should roughly stick together when pinched between two fingers. If the mixture is still a bit dry, add some extra liquid sweetener or water and mix until the ingredients come together. Firmly press the mixture into the bottom of the lined cake tin. Bake the crust for 10-15 minutes or until it is golden brown.
To make the filling: Add all ingredients to a medium-size saucepan. Bring the mixture to a boil while whisking constantly. Reduce the heat to medium and let it simmer for 10 minutes or until slightly thickened. If your custard has any lumps, blend it with a stick blender until smooth.
Pour the filling onto the crust and allow it to cool at room temperature for at least 1 hour. When it has cooled, place the slice in the fridge to set further.**
When you are ready to serve the bar, remove it from the tin. Spread the passion fruit puree on top and cut. The slice can be kept in an airtight container in the fridge for up to 3 days.
* Black salt is an Indian ingredient which has a pungent egg-like smell. You can get it from Indian/South Asian shops or health-food stores.
** The custard needs to gradually cool down otherwise it will crack.