1.5 oz. light rum (like Plantation 3 Star)
1.5 oz. fresh grapefruit juice
1 oz. lime juice
1 tsp rich simple syrup (2:1)
1 dash aromatic bitters
Combine all ingredients in a shaker with ice and shake until well-chilled. Double-strain into a chilled coupe and garnish with a grapefruit peel.