In a cocktail shaker, combine whiskey, lemon juice, simple syrup, and egg white (if desired).
Shake the mixture vigorously without ice for 15-20 seconds to blend the egg white and achieve a frothy consistency.
Add ice to the cocktail shaker and shake once more for another 15-20 seconds to cool the beverage.
Strain the mixture into a rocks glass filled with new ice. Garnish with a lemon slice and/or maraschino cherry. Optionally add drops of Angostura bitters to the foam and use a toothpick to make a pattern.