Pour liquors into a pitcher. Fill with ice, add bitters, and stir rapidly until very cold, 30 to 40 times.
Strain cocktail into 10 coupe or martini glasses. Top each with grapefruit zest; spritz with grapefruit oil if you like.
*Find grapefruit essential oil at aftelier.com.
Make ahead: Liquors in a sealed mason jar, up to 3 days, chilled.