Add the aged rum, light rum, fresh lime juice, orange curacao, and orgeat syrup to a cocktail shaker with ice.
Shake vigorously for about 15 seconds until well chilled.
Strain into a rocks glass filled with crushed ice.
If desired, float the dark rum on top by slowly pouring it over the back of a spoon.
Garnish with a fresh mint sprig, lime wheel, and orange slice. Serve immediately.