Technique
If using egg white, shake all ingredients without ice first to build foam, then add ice and shake again to chill and dilute. Without egg white, skip the dry shake and shake once with ice; the extra unchilled step offers no benefit.
A whiskey sour is a mixed drink containing whiskey, lemon juice, sugar, and optionally, a dash of egg white.

TonicDB editorial guide
This Whiskey Sour uses a clear 2:1:½ structure: whiskey leads, lemon supplies acidity, and syrup softens the edges. Egg white is optional; it changes the texture and creates a foam cap but does not define the drink’s basic sweet-sour balance.
If using egg white, shake all ingredients without ice first to build foam, then add ice and shake again to chill and dilute. Without egg white, skip the dry shake and shake once with ice; the extra unchilled step offers no benefit.
Strain over fresh ice in a rocks glass, leaving behind the small chips from the shaker. A lemon peel and cherry are traditional, but the foam and whiskey aroma are more important than a large garnish.
A short second shake can leave an egg-white version warm beneath its foam, while too much syrup covers the whiskey. Fresh lemons vary, so taste the mixture and adjust by a bar spoon instead of changing the recipe in large jumps.
For an egg-free foamed sour, use a small amount of aquafaba and follow the same dry-shake method. For a drier drink, reduce simple syrup to a quarter ounce; for more depth, float a small spoonful of dry red wine on the finished sour.
These notes explain how to execute and adapt the recipe shown above. Ingredients and serving traditions vary, so taste and adjust thoughtfully.
Contributed by @alfg