Halve each ripe passion fruit and scoop the pulp and seeds into a blender.
Add 480ml cold water and the caster sugar, then blend on high for 30-60 seconds, until the pulp separates from the seeds.
Place a fine sieve over a jug and pour the mixture through, pressing down on the solids to extract the juice. Discard the seeds and pulp.
Stir in the remaining 240ml cold water. Taste and add more sugar or water if needed.
Chill in the fridge for at least 30 minutes, or serve over ice straight away.
Garnish with fresh mint sprigs if you like.