Combine ingredients: In a cocktail shaker, add amaretto, lemon juice, simple syrup, and egg white (if using).
Dry shake: Shake vigorously without ice for about 15 seconds to emulsify the egg white and create a silky texture.
Shake with ice: Add ice to the shaker and shake again for another 15–20 seconds until well chilled.
Strain: Strain into a rocks glass filled with ice.
Garnish: Top with a cherry or a twist of lemon peel, and serve