There is much to support the Old Fashioned's claim to be the original cocktail. In fact if you take your definition of 'cocktail' from The Balance and Columbian Repository of Hudson, New York (and frankly, where else are you getting your definition of cocktail from?) it neatly fits the criteria: liquor (in this case whiskey), bitters, sugar and water (in this case frozen).
Like most cocktails of the nineteenth century, you can take your pick of Old Fashioned origin stories. My favourite is that it was conceived at The Pendennis Club, Louisville, Kentucky in the late nineteenth century, popularised by the fittingly named local bourbon distiller Colonel James Pepper and taken to New York where it grew to be enjoyed by such luminaries as Sterling Cooper's own Don Draper.
Whatever its vintage, however, the Old Fashioned is a punchy cocktail for those who hold no truck with paper umbrellas and sparklers getting in the way of their drinking. The exact composition is something that enthusiasts can argue about for days. Does an Old Fashioned require rye or bourbon? A muddled orange? A cherry? The earliest known recipe (dating from 1895 no less) instructs:
Dissolve a small lump of sugar with a little water in a whiskey-glass; add two dashes bitters, a small piece ice, a piece lemon-peel, one measure whiskey. Mix with small bar-spoon and serve, leaving spoon in glass. - Kappeler (1895). Modern American Drinks: How to Mix and Serve All Kinds of Cups and Drinks.
It's a simple drink, but essays have been written about the correct (and myriad incorrect ways) to make an Old Fashioned. Most boil down to personal preference, but if you diverge too far from whiskey, sugar, water and bitters you're probably making a delicious Old Fashioned variant. But that's ok (unless you're adding soda water or lemonade) because this is one of the core purposes of the Old Fashioned. The template is malleable and will stand up to plenty of adaptation, which is why a house-style Old Fashioned is on the menu at pretty much every bar in the western world.
Before we get there, let's start with the basic recipe and technique:
THE OLD FASHIONED
Take a mixing glass and add a barspoon of simple syrup, 2-3 dashes of bitters, a handful of ice and 30ml whisky.
Stir for one minute.
Add another 30ml whisky and another handful of ice.
Stir for one minute.
Strain into a chilled rocks glass over fresh ice.
Garnish (typically orange peel for bourbon, lemon peel for rye).
A few notes for the advanced student:
That's the simple premise of your basic Old Fashioned. But as with most vices, the devil's in the details:
Ice
Ice is important in every cocktail, but in a drink as unforgivingly simple as the Old Fashioned it becomes critical. You should be using more ice than you would think you need. This is because the key to the drink is the dilution. Many people assume more ice means more water in the drink, but in fact, the reverse is true. Use plenty of ice and it stays colder for longer, limiting the dilution and giving you a better chance of a more balanced drink.
Sugar
Traditionally the drink was made with a bar spoon of sugar combined with a bar spoon of water, or later a sugar cube and a bar spoon of water. If you really want to find a sense of zen-like peace from your Old Fashioned making then this is a fine way to go about it. If however, you want a balanced presentable drink as quickly as possible, sugar syrup is a better option. Recipes for sugar syrup abound and it also provides our first opportunity to craft a variant through the use of flavoured syrups. Before that, though you need to decide on whether you want a classic 1:1 sugar and water ratio, or a richer 4:3 ratio, and of course what sugar you're using. This will depend on how sweet or dry you like the drink - the richer the syrup, the less you need for optimal sweetness and the less water you're adding. There's a trend for bartenders to use brown sugar syrups when mixing with brown spirits, and this will also increase the richness of flavour. Ultimately, it's your drink, so your call.
Bitters
Another personal choice and another balancing act. There are thousands of brands and flavours of bitters available these days. A classic Old Fashioned will probably use Angostura or another 'aromatic' style bitters, or possibly orange (which will harmonise nicely with the garnish). Beyond that you run the risk of adding an extra flavour to the drink. You might want that, but then you're back in variant territory. On the flip side, the unforgiving template of the Old Fashioned is a great way to determine if your bitters are up to scratch. I've been working on homemade cola bitters for w while now, and if they don't impart the right flavour in an Old Fashioned, they're not ready.
Spirit
American whiskey is the classic (although for stylistic reasons I've referred to 'whisky' in my recipe). Bourbon is a popular choice, although there's a compelling argument that rye would have been the traditional choice pre-Prohibition. Again, personal choice plays its part here. You can't go wrong with a classic such as Maker's Mark or Woodford Reserve. Elijah Craig Small Batch is also an excellent choice. Beyond these, there are hundreds of bourbons available now with a range of mash bills - meaning some have higher rye (pepper, spice), or barley (toffee, cereal) content. That might be what you're looking for, if not, stick to a corn dominant bourbon for the classic vanilla, maple syrup profile.
Glass
The Old Fashioned is one of the few drinks to give its name to a glass. Usually a heavy-bottomed tumbler, or a large rocks glass it should have some heft and be large enough to hold 60-90ml of liquid and a rim-reaching quantity of ice. Just as in the mixing, the ice in the glass is an important factor. You want to manage the dilution of the drink after serving as well. If you skimp on the ice it will melt too fast and you will lose the flavour of the drink before the bottom of the glass. You want enough ice so it reaches the rim, doesn't float and doesn't shift around and bop you on the nose as you drink.
Technique
The two-minute split stir should be sufficient, with the right quantity of ice, to leave the drink slightly underdiluted at the point of serving. This is important because it will continue to dilute in the glass. You want to manage the journey through consumption of the Old Fashioned so that the first sip is just tough enough to remind you you're just a hair's breadth away from drinking neat whisky, but the last sip isn't just the faint memory of the spirit.