The unveiling of a new fine spirits release is often touted as a landmark event; almost as often, that is an overstatement. A new Louis XIII Rare Cask edition, though, is a true rarity. The latest, Rare Cask 42.1, is only the third ever bottled.

The first (Rare Cask 43.8) was bottled by former cellar master Pierrette Trichet – the contents of one particular tierçon (the huge Limousin oak barrels in which Louis XIII matures). She referred to it as a discovery: a single cask so outstanding that she would go against the natural inclination of a maître de chai (cellar master) – Always. Be. Blending. – and let it develop until perfect in isolation.

Trichet discovered the cask in 2004, let it mature another five years, then bottled it in 2009. The next discovery was not revealed to the world until 2013: Rare Cask 42.6. By that time, Trichet was preparing to retire and collaborated with her apprentice and successor Baptiste Loiseau. A decade on, he has found a tierçon he is prepared to bottle as the third Rare Cask. The designation 42.1 – as with its predecessors – refers to the cask strength, which is higher than the usual 40 per cent ABV of Louis XIII.

That is not the only point of elevation, however. Each stage of the production of Louis XIII is tightly controlled, with a view to quality and consistency in a drink of depth, complexity and finesse. This starts in the vineyards and wine presses of Grande Champagne, Cognac’s premier cru, and continues into a very particular distillery method, which includes a second distillation of the eau de vie, this time on fermentation lees, adding a certain viscosity and additional aromas.

Most of all, quality and consistency are achieved by ageing the eaux de vie in differently aged casks (some tierçons are over a century old), located in drier or damper parts of the cellars. The cellar master is constantly “marrying” (blending) casks as they age, so that the final decanter contains liquid originally distilled across several decades.

Given that craft, it is quite something when the cellar master identifies a cask that needs no blending and can be regarded as Louis XIII but turned up to 11 (or, in this case, 42.1). Because, while outstanding, Rare Casks are not outliers… they are classic Louis XIII with an emphasis on certain tasting notes.

The Rare Cask 42.1 filled 775 Baccarat decanters: black crystal versions (with a gold and rhodium neck band) of the distinctive design that Paul-Émile Rémy Martin based on an antique metal flask found on a battlefield in Jarnac, in the Cognac region. Each is accompanied by two special editions of the “Facets of Louis XIII” glasses and a beautiful pouring pipette.

The only place in the UK you can acquire a set will be the Louis XIII boutique in Harrods. The good people at Louis XIII prefer to talk of “enquiries”, but other sources suggest it will be priced at £47,000. Should you require a tasting before committing to a whole decanter, the locations are equally limited.

The Dorchester has long had a relationship with Louis XIII and the new Vesper Bar houses an elegant trolley for a quite superb tasting experience. Currently, The Dorchester offers an experience involving three limited-edition decanters: a Black Pearl Anniversary Edition, a Time Collection II: City Of Lights – 1900, and one of the Rare Casks.

From the beginning of June, Vesper will be the only place in the UK – and, because few establishments have been wise enough to hoard the previous Rare Casks over the decades, possibly the world – where you can taste all three Rare Casks.

The tasting flight is accompanied by food pairings designed by Martyn Nail, culinary director of The Dorchester, to highlight various flavour profiles of the cognac. And, with head bartender Lucia Montanelli using a specific pipette to serve from each decanter, there is a feeling of ceremony about the experience.

One surprising detail in the ritual is that you are encouraged to clink glasses. The facets around the epicure (where the bowl meets the stem) are where two of these glasses should meet, producing a beautifully resonant chime – in G sharp, to be specific.

Angling the glass to clink serves another purpose, too: it provides enough of a swirl to free up the lighter aromas of Louis XIII. There’s no need for vigorous swirling with a cognac of this quality; even from a distance, the floral notes notes are detectable – honeysuckle, jasmine, a hint of violet. As you bring your nose closer, richer notes of sultana and pecan, along with burnished wood, come to the fore.

On the palate, there’s a surprising freshness – lemon balm and mint – to the first sip, before the smooth liquid coats your mouth and offers up dried fruit flavours, notably plum, as well as honey and leather, then finishes with lingering, gentle spiciness.

Rare Cask 42.1 is said to have all of these flavours in harmony, but with the floral notes enhanced and maintained from nose to finish. Louis XIII private clients can arrange experiences which begin with a tasting at The Dorchester and continue with a visit to the Rémy Martin family estate.

louisxiii-cognac.com; dorchestercollection.com