This warm mixture is a distinctive twist on the classic hot buttered rum.
- Using a muddler or large wooden spoon, mash sugar and herbs together in a large saucepan until the herbs are bruised and fragrant.
- Add water and bitters; bring to a simmer over medium-high heat, stirring often, about 5 minutes. Stir in rum and cook for 1 minute. Add butter and cook, stirring, until it melts.
- Strain the mixture into a pitcher. Let stand for about 10 minutes to mellow the flavors. Serve in cordial glasses or other small heat-safe glasses, garnishing each drink with a mint leaf.