The elegant Cuban combo of lime and rum has nothing to do with the syrupy slush you’ll find at swim-up bars.
Combine sugar, rum, lime juice, and simple syrup in a cocktail shaker; stir until sugar dissolves, about 20 seconds. Fill shaker with ice, cover, and shake vigorously until outside of shaker is very cold, about 20 seconds. Place a large ice cube in a coupe glass; strain cocktail through a Hawthorne strainer or a slotted spoon into glass.
Using a small serrated knife, remove a 1" strip of peel from lime (some white pith is okay); it should be stiff enough to provide some resistance when bent. Twist over drink to express oils; discard. Garnish with a lime twist.
1 bar spoon (or 1 teaspoon) superfine sugar
1¾ ounces white rum
¾ ounce fresh lime juice
½ ounce simple syrup