Add the whiskey, fresh lemon juice, simple syrup, and egg white to a cocktail shaker.
Dry shake (without ice) vigorously for about 15 seconds to emulsify the egg white.
Add ice to the shaker and shake again until well-chilled.
Strain the mixture into a rocks glass filled with ice.
Garnish with a lemon twist or wheel and a maraschino cherry.
Serve immediately and enjoy your Whiskey Sour.