A Scotch and vermouth cocktail, this is closest to a perfect Rob Roy, which can be made with two sets of proportions..
Scotch doesn’t find itself in a lot of cocktails because of the characteristics of the spirit, and the wide range it can present depending on the region of production for single malt scotches, or the key malts chosen for scotch blends. Some Scotch exhibits peat notes, others exhibit more malt notes, while others demonstrate floral or honey notes, or some combination of the aforementioned along with other flavors such as citrus, white fruits, spice, or more grain notes. In the case of the Affinity cocktail, which uses Scotch, the choice of Scotch is pretty important, and choosing one that will add depth to the drink, rather than just being absorbed by the vermouth, is substantially important. As of such, I recommend the utilization of a blend, to limit the cost of the cocktail, but also some blend with a lot of peat, to give a bit of a smokey element to the drink, or perhaps one that has more floral notes to contrast against the herbal notes of the vermouth.
Classified as a Scotch cocktail of the aromatic type by Embury, the Affinity according to Embury is a Medium Manhattan with Scotch in place of rye or bourbon (176-178). The terminology Medium Manhattan comes from the idea of using equal proportions of types of vermouth, blending between the dry and the sweet to produce a balanced flavor (Ibid 122). For Embury, the ratio of spirit to vermouth in a Manhattan is two to one, meaning that in the case of the Affinity, the drink should be made with a quarter of a part of each vermouth in ratio to that of the Scotch. Proportions, just like most drinks, vary depending on source, most certainly due to historical circumstances and contexts but also on account of preference. In other texts, such as Craddock’s, the Affinity is an equal proportions of each ingredient (less the bitters), which makes for a really strong vermouth driven drink (Craddock 17); personally I prefer the version that Embury sets out, which is also the same as set out by Frank Meier (24).
Whatever you do, make sure to include the garnish for this drink: the lemon peel really sets off the flavor of the Scotch, especially if the key malt is one from the highlands with more citrus oriented notes. The aromatics also help to provide a bit more complexity to the drink which would be predominately driven by the flavors of the herbs in the vermouth.
1 1/2 ounces Scotch
1/3 ounce dry vermouth
1/3 ounce sweet vermouth
2 dashes Angostura bitters
Stir over cracked ice, straining into a cocktail glass, and garnish with a lemon twist.
Craddock, Henry. 1999. The Savoy Cocktail Book. Originally published 1930. London: Pavilion Books.
Embury, David A. 2009. The Fine Art of Mixing Drinks. Originally published 1948. New York: Mud Puddle Books, Inc.
Meier, Frank. 1936. The Artistry of Mixing Drinks. Reprint of original. Paris: Fryam Press.