Add the rum, lemon juice, simple syrup, and egg white to a cocktail shaker without ice.
Shake hard for 15 seconds - this is the dry shake that creates the foam.
Add a generous handful of ice and shake again for 15 seconds until well chilled.
Double strain into the chilled coupe glass.
Add a few drops of Angostura bitters on the foam and drag through with a cocktail stick.
Garnish with a dried orange or lemon wheel.
Notes
Double strain through a fine mesh sieve so the foam sits clean on top.Golden rum gives a lighter, more balanced result. Dark rum gives something richer and more intense.Dried citrus garnish. A dried lemon or orange wheel rested on the foam looks really beautiful and is the perfect garnish for a sour. You can buy them or make your own by slicing citrus thinly and drying in the oven at a low temperature (around 90C / 200F) until completely dry.