Fill a cocktail shaker two-thirds full of ice.
Add the white rum, fresh lime juice, orange curaçao and orgeat syrup.
Put the lid on and shake vigorously for 15–20 seconds until well chilled.
Fill an old-fashioned glass with ice.
Strain the mixture from the shaker into the glass over the ice.
Slowly float the dark rum on top by pouring it over the back of a spoon held just above the surface of the drink.
Garnish with a sprig of mint (gently clap between your hands to release the oils) and a wheel of lime.