Add bitters, simple syrup, lemon juice, whiskey, and aquafaba to a cocktail shaker or mason jar.
Cap the cocktail shaker tightly and dry-shake vigorously for 30 seconds without ice.
Add ice and shake again, starting slowly and then increasing speed, until well-chilled. You should feel the liquid slightly thicken as you shake.
Strain into a coupe glass served up, or strain into a rocks glass over ice.
Garnish with a dash of Angostura bitters and a Maraschino cherry, if desired.