Chill a large snifter with ice and set aside.
Add the white rum, cabbage rum, triple sec and simple syrup to a shaker with ice, and shake vigorously for 15 seconds.
Strain into the chilled snifter.
Add 3 oz sparkling water.
In a separate container, combine 3/4 oz grapefruit juice and 1/2 oz lime juice. Pour the juice into the snifter.
Add 1 pinch salt.
*Cabbage rum: Combine 1/2 head purple cabbage, chopped, with 4 oz white rum in a blender, and blend on high until smooth. Strain using a fine-mesh strainer. Makes 4 oz.