In a shaker, dissolve the sugar cubes in 1 ounce of the club soda. Add the cardamom and muddle. Add the rum, lime juice, grapefruit juice and bitters, one at a time, while stirring. Fill with ice and stir. Strain into a cocktail or punch glass and top with the remaining 2 ounces of club soda.Garnish with lime wheels.
Base Spirit :