Prepare the Lime Syrup: Combine lime wheels, sugar, and water in a small saucepan over medium-high. Bring to a simmer, stirring often until sugar dissolves, about 1 minute. Remove from heat, and let cool to room temperature, about 30 minutes. Remove and discard lime wheels, reserving syrup. Refrigerate syrup in an airtight container for up to 2 weeks.
Prepare the Cocktail: Fill a cocktail shaker with ice; add dark rum, overproof rum, lime juice, light rum, Cointreau, Lime Syrup, and powdered sugar. Cover with lid, and shake vigorously until thoroughly chilled, about 15 seconds. Strain into a large highball glass filled with fresh cracked ice. Add more Lime Syrup, if desired. Garnish with a lime wheel. Serve immediately.
Excerpted from Beach Cocktails by the Editors of Coastal Living. © 2017 Oxmoor House.