Negroni Week lands on 21 September this year, which is either a coincidence or the drinks calendar showing rare good sense. Just as London gets its first properly cold evening, the world's best bars start pouring bitter red aperitivo by the case. The Negroni takes the billing, as it always does. But the drink you actually want once the clocks start thinking about changing is its warmer, whiskey-drinking cousin: the Boulevardier.

The Boulevardier is what happens when a Negroni swaps its gin for bourbon and puts on a heavier coat. Same bitter backbone, same ruby glow, but where the Negroni is crisp and pine-sharp, this one is deep, spiced and quietly autumnal. It is the single best way to keep drinking Negronis once the weather turns, and almost nobody outside the trade seems to know it is there for the taking.

A Negroni in a heavier coat

The drink is named after a man and a magazine. In 1920s Paris, an expatriate American writer called Erskine Gwynne edited a society journal named the Boulevardier, and he took his Campari with American whiskey rather than gin. The recipe first turns up in print in 1927, in Harry McElhone's Barflies and Cocktails from Harry's New York Bar on the Rue Daunou, alongside a dry note that Gwynne was drinking rather a lot of them. A century on, that first appearance falls almost exactly now, which is as good an excuse as the season itself.

Think of the whole family as three variations on one very good idea. Gin gives you the Negroni, all juniper and cool composure. Bourbon gives you the Boulevardier, rounder and warmer and better suited to a fire. The bitterness and the ruby colour stay put; only the spirit at the centre changes character.

The recipe, and the one tweak worth making

Equal parts is the honest starting point, exactly as with a Negroni: one part bourbon, one part Campari, one part sweet vermouth, stirred over ice and finished with a twist of orange. It works. But whiskey has more to say than gin does, and poured in equal measure the Campari can bully it into silence. The fix, which most good bars now make quietly, is to give the whiskey a little more room.

Try 45ml bourbon to 30ml each of Campari and sweet vermouth. The drink keeps its bittersweet edge but gains a warm, boozy centre for the whiskey to stretch out in. That is the version worth learning by heart. Everything else is a matter of which bottles you reach for.

Bourbon, or rye?

The whiskey is where you make the drink your own. A classic Kentucky bourbon keeps things round and sweet, all vanilla and baked apple, and folds into the Campari as though the two were introduced years ago. Buffalo Trace is the one we reach for most: honest, softly spiced Frankfort bourbon at a price that does not flinch when you pour it into a cocktail. If you want more oak and a touch more proof, Elijah Craig Small Batch pushes the whole thing darker and more brooding. And the wheated W.L. Weller Special Reserve makes the softest, most dangerously sippable Boulevardier of the lot.

Rye is the other road, and a very good one. Where bourbon soothes, rye argues, and its pepper cuts through the Campari's sweetness beautifully. Sazerac Rye turns the Boulevardier crisp and spicy, and closer in spirit to how the thing was probably drunk in the first place. Use rye and you are within a whisker of a related drink called the Old Pal, of which more in a moment.

The other two thirds

Everyone fixates on the whiskey, but the drink is really made by the parts that never change. Campari is non-negotiable. That vivid, bittersweet red, first mixed by Gaspare Campari in Milan in 1860, is the drink's entire personality: orange peel, rhubarb, and a bitterness that leaves your mouth watering for the next sip. Reach for a lesser bitter and you will taste the compromise straight away.

The sweet vermouth is where a Boulevardier quietly gets classy. Carpano Antica Formula is the benchmark, made much as it was when Antonio Benedetto Carpano more or less invented vermouth in Turin in 1786: rich, vanilla-laced and faintly cola-like, it gives the drink a plush middle that a Negroni's lighter frame never asks for. If you would rather not commit to a full litre, Dolin Rouge from Chambéry is lighter and more delicate, and does the job with real grace.

Get those two right and the whiskey almost cannot fail. If you are stocking a shelf from scratch for the season, start with the bittersweet heart of the thing and build outward.

Between those two you have most of the drink in your hands. The bourbon brings the warmth, the Campari brings the backbone, and a bottle of sweet vermouth from the shelf turns the pair into something you will want every evening from now until spring.

Building it properly

Stir, do not shake. Shaking bruises a spirit-forward drink and clouds it, and you want this one glossy and clear. Fill a mixing glass with good, hard ice, pour your three (or three-and-a-bit) parts over the top, and stir for longer than feels necessary, thirty seconds at least, until the outside of the glass is cold enough to sting your fingers.

Strain over one big cube in a rocks glass, or up into a chilled coupe if you are feeling formal. Then the orange. Twist a wide strip of peel over the surface so its oil catches the light, run it round the rim, and drop it in. That last flourish is not decoration; it is the smell of the drink, and it matters far more than people give it credit for.

One drink along: the Old Pal

Make the same drink with rye instead of bourbon and dry vermouth instead of sweet, and you have an Old Pal: paler, drier and more bracing, a Boulevardier that went out for a cold walk. It is a useful thing to have up your sleeve when someone finds the sweet version too rich. All of these live in the same family as the Negroni itself, the gin original that started the whole conversation. Learn the three and you can read almost any bittersweet drink a bar puts in front of you.

So let Negroni Week do its work, by all means. But when the evening turns and the gin starts to feel like it belongs to a warmer month, reach for the bourbon instead. The Boulevardier is the Negroni's answer to autumn: same red, same bitterness, a good deal more fireside. Pour one, cut the orange, and settle in for the long half of the year.