Add sugar and lime juice to a cocktail shaker. Stir well to help the sugar dissolve. Let the mixture sit for ~10 minutes, giving it a stir every few minutes, until the sugar is dissolved.
Fill the cocktail shaker halfway with ice. Add in the rum and passion fruit juice. Shake for 10 seconds.
Strain into 2 cocktail glasses. Optional: scoop the pulp out of half of a passion fruit into each glass.