Combine white rum, fresh lime juice, and superfine sugar in a cocktail shaker.
Stir briefly without ice to dissolve the sugar completely.
Add a generous scoop of ice and shake vigorously for 10–12 seconds until well chilled.
Double-strain into a chilled cocktail glass.
Serve immediately with no garnish.
Superfine (caster) sugar is specified in the IBA recipe, but many bartenders substitute a 2:1 simple syrup for more consistent integration - if using sugar, dissolve it thoroughly before adding ice to avoid graininess in the final drink.
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