Chill a coupe or martini glass by placing it in the freezer for a few minutes or filling it with ice water while you prepare the cocktail. Discard ice water just before straining.

Dry shake (if using egg white)

Add the egg white (if using), lime juice, simple syrup, cassis, activated charcoal (if using), and bitters to a cocktail shaker. Dry shake without ice vigorously for 15–20 seconds to emulsify and build foam.

Wet shake

Add ice cubes to the shaker, then pour in the black rum. Secure the lid and shake vigorously for 12–15 seconds until the shaker is very cold and the mixture is well aerated. If you did not use egg white, simply combine all liquid ingredients with ice and shake for 12–15 seconds.

Fine strain

Double-strain the cocktail through a fine-mesh strainer into the chilled coupe or martini glass to remove ice shards and any coagulated egg white solids. The drink should be deep, near-black in color; if it appears too thin, consider an additional 0.25 oz simple syrup next time.

Garnish

Float a lime wheel or a thin lime twist on the surface of the foam or place it on the rim. Optionally top with a single dehydrated blackcurrant or blackberry placed gently in the center of the foam for contrast.

Serve immediately. Advise the drinker to stir lightly before sipping to blend the floated garnish and foam into the cocktail.