Add the dark rum, orgeat, lemon juice, egg white and bitters to the cocktail shaker or mason jar without ice. Seal and shake for 30 seconds. This is the dry shake.
Open the shaker or jar and add a big handful of ice. Seal and shake vigorously for a minute or two. This is the wet shake, and it's a longer shake to properly build the topping froth.
Strain the rum sour into a chilled sour, coupe or martini glass. Garnish with a lemon twist if desired. Toast and sip away.
There isn't a more perfect drink for rum lovers, and you'll just love how the egg white gives this drink the perfect creamy topping. Cheers!