Add white rum, fresh lime juice, pineapple juice, and simple syrup (plus dark rum if using).
Shake hard for 10–15 seconds until the shaker feels very cold.
Fine strain into a chilled coupe or martini glass.
Garnish with a pineapple wedge or lime wheel and serve immediately.
Notes
Fresh lime juice brightens the drink and balances the pineapple. If too tart, add a dash more syrup; if too sweet, squeeze in extra lime. For a frozen version, blend all ingredients with crushed ice instead of shaking.