Aged white rum seems like an oxymoron, but it adds an edge to this beach vacation of a cocktail.
Preparation
Muddle lime and brown sugar in a cocktail shaker. Add grapefruit and kumquat and muddle until lightly crushed and juices are released. Fill cocktail shaker with ice, add rum, and shake vigorously until very cold, about 20 seconds. Pour into an old-fashioned glass or tumbler and serve with two straws.
