Cut open the passion fruit and push through a fine mesh strainer to catch all the juice, press hard against the passion fruit to also catch the pulp. Get as much juice and pulp from the passion fruit as possible. Reserve the seeds. Set the juice aside.
In a cocktail mixer, muddle the mint and the sugar together to help release the oils from the mint. Add the rum, lime juice, and ice, then shake vigorously.
Add ice to your glass (preferably a high ball, but not required) and divide the mixture among the glasses. Top with a splash or two of club soda and stir in the passion fruit juice/pulp. Top with mint and passion fruit seeds for garnish (can eat passion fruit seeds)