In a mixing glass, combine the gin, vermouth, and Fernet Branca. Fill the glass with ice and stir until chilled.
Strain the cocktail into a chilled Nick and Nora glass or two, if you have them, and garnish with the orange twist.
Notes
Split the drink--This makes a very stiff drink, which would be considered a double. If you want to consume less alcohol, divvy the drink between two glasses and share.
Add some OJ--Modern versions of the cocktail include a splash of orange juice. To try this, add a splash with the other ingredients in step 1.