I like to add a dash of orange bitters; the slight fruitiness tames the sharper edges of the drink.
The recipe comes from Ada Coleman, who was the head bartender at The Savoy Hotel in London in the early 1900s. According to The Savoy Cocktail Book, the drink was originally shaken, but most people prefer to stir, rather than shake.
1 1/2ouncessweet vermouth
Add gin, vermouth, Fernet Branca, and orange bitters (if using) to a cocktail mixing glass. Fill 2/3rds full with ice and stir briskly until very cold, about 15 seconds.
Strain the mixture into a cocktail glass and garnish with an orange twist.