Take the leaves from four mint sprigs and put them in a cocktail shaker. Add the simple syrup and lime juice and then gently bruise the mint with a muddler.
Next, pour in Demerara rum (we are partial to El Dorado, for a traditional but inexpensive Guyanese rum) and swirl the shaker to combine the rum and the mint.
Pour the shaker's contents into a Collins glass and add crushed ice almost to the top, then add six dashes of angostura bitters.
This is the tricky part. The drink gets its name from the method in which it's made—it's swizzled rather than stirred or shaken. Real swizzle sticks are hard to come by, so a bar spoon will have to do. Slide the bar spoon into the glass, stopping just before the mint leaves at the bottom. Rub your palms together, gently rotating the bar spoon and thus "swizzling" your drink. Ideally, the bitters will remain at the top, while the mint remains at the bottom, leaving a beautiful, three-layered libation.