Combine gin, lemon juice, blackberry brandy, egg white, heavy cream, and granulated sugar in an empty cocktail shaker. Shake vigorously for 15 seconds (make sure the top to the shaker is on nice and tight!).
Fill the shaker halfway with ice, add the top back on, and shake again for about 10 seconds. Pour into a highball glass and fill with club soda, stopping just short of the top of the glass. Wait for the foam to rise before serving. Garnish with a fresh blackberry.