From Beachbum Berry Remixed, via an article in Serious Eats July 9 2010.
3 oz dark Jamaican rum
1 oz simple syrup
3/4 oz fresh lime juice
3 dashes Angostura bitters
Combine ingredients in tall glass and fill with crushed ice. Swizzle with a bar spoon until a frost forms on the outside of the glass. The ice will settle as you do this; add more ice to fill and garnish with a mint sprig.