Add the rum, lime juice and syrup to a shaker with ice, and shake until well-chilled.
Drop the mint sprig in the bottom of a Collins glass, add fresh ice, and strain the drink on top.
Garnish with a mint sprig, and slowly pour the liqueur onto the leaves and the top of the drink.
*House-spiced rum: Add 1 stick of cinnamon, 5 ground cloves and 3 ground star anise to 1 750mL bottle of aged rum. Let steep for at least 12 hours or until desired flavor is achieved, then strain out solids.