This is one of those rare cocktails which doesn't feature one of the six base spirits (brandy, gin, rum, tequila, vodka, whiskey). Benedictine carries the day here, and since it is 80 proof like a base spirit I suppose it's just a slight deviation.
Garnish: Orange twist.
Stir with ice.
Strain into a cocktail glass.
Source: The Savoy Cocktail Book (1930)