Prepare the coconut syrup. In a small saucepan, combine sugar, water, and coconut milk. Bring the mixture to a simmer and stir until all the sugar dissolves. Remove from heat and allow the mixture to cool down.
Add 4 mint leaves, rum, lime juice, and coconut syrup into a highball glass. Crush the mint using a muddler to release mint oils.
Add ice and top with club soda. Stir. Garnish with a mint sprig, lime slices, and fresh coconut. Enjoy!
Store the leftover coconut syrup in an airtight container in the refrigerator. The syrup can be stored up to 1 week.