Chill a highball or Collins glass by filling it with ice water. Let sit while you prepare the cocktail, then discard the ice water and add 4 fresh ice cubes for serving.

Dry shake (emulsify)

Add the gin, fresh lemon juice, fresh grapefruit juice, simple syrup, and the egg white to a cocktail shaker (no ice). Secure the lid and dry shake vigorously for 15–20 seconds until the mixture becomes frothy and slightly thickened. This step helps create a stable foam.

Wet shake (chill)

Open the shaker, add 6 ice cubes, reseal, and shake hard for 10–15 seconds until the shaker is cold to the touch (this chills and dilutes the mixture).

Strain

Discard the ice water from the chilled glass if you used it. Double-strain the shaken cocktail into the prepared glass over the 4 fresh ice cubes using a fine mesh strainer and the shaker's strainer. Double-straining removes small ice shards and any bits of coagulated egg.

Top with soda

Gently pour 2 oz club soda down the side of the glass to preserve the foam. Pour slowly to avoid breaking up the head.

Garnish and finish

Express the oils from a grapefruit twist over the surface of the drink to add aroma, then drop the twist or a thin grapefruit wheel onto the foam. If desired, add 1–2 dashes of Angostura bitters on the foam and use a toothpick to create a small decorative swirl.

Serve

Serve immediately while the foam is still fresh. Sip and enjoy the bright citrus balance with effervescence and a creamy foam.