In a cocktail shaker, add 1 cup cubed ice, 2 oz aged light or gold rum, 0.5 oz simple syrup, 0.75 oz fresh lime juice, 0.25 oz fresh lemon juice, and 2 dashes Angostura bitters. Secure the shaker lid and shake vigorously for 10–15 seconds until the shaker is well chilled (exterior will feel cold).

Prepare glass

If using crushed ice, fill a rocks glass about three-quarters full with crushed ice (or add fresh cubed ice if preferred).

Strain and float

Double-strain the shaken cocktail into the prepared glass over the ice using a Hawthorne or fine mesh strainer. Gently pour 0.25 oz dark rum over the back of a spoon so it floats on top of the drink, creating a layered look.

Garnish and serve

Garnish with a lime wheel or wedge on the rim and an optional mint sprig slapped once to release aroma and placed alongside the lime. Serve immediately with a short stirrer or straw so the drinker can incorporate the float if desired.