If using egg white, separate the egg and reserve about 1/2 of the white (approximately 1/2 unit). Chill a coupe or rocks glass in the freezer or fill with ice water to chill while you mix.

Dry shake

Add rye whiskey, fresh lemon juice, fresh grapefruit juice, simple syrup, orange liqueur, Angostura bitters, and the egg white (if using) into a cocktail shaker without ice. Seal the shaker and shake vigorously for 10–15 seconds to emulsify and create foam.

Wet shake

Open the shaker and add a generous scoop of ice (use the ice for shaking). Seal and shake vigorously for 15–20 seconds, until the shaker feels very cold to the touch and the outside is frosty.

Double strain and serve

Discard ice or water from the chilled glass. Double-strain the cocktail into the chilled glass using a hawthorne strainer and a fine mesh strainer to remove ice shards and ensure a silky texture. If serving on the rocks, strain over fresh large ice in a rocks glass instead.

Garnish

Express the oils of a grapefruit twist over the surface of the drink by holding the twist skin-side down about 2–3 inches above the cocktail and giving it a quick snap. Rub the peel's rim on the glass (optional) and drop the twist into the drink or float a thin wheel on the foam. Finish with a light dusting of grated nutmeg or a microplane of citrus zest if desired.