You’ve heard of Brown’s, naturally. It was the place Alexander Graham Bell made London’s first telephone call; where Agatha Christie was a guest – finding inspiration for At Bertram’s Hotel in the process; heck, it was even where Theodore Roosevelt stayed before getting married.

And if you’ve heard of Brown’s, you’ve also probably heard of Donovan Bar, the swish hotel bar named after Swinging Sixties photographer Terence Donovan. And if you’ve heard of Donovan Bar, then chances are you’re familiar with Salvatore Calabrese – AKA The Maestro, inventor of the breakfast martini and holder of the world record for the most expensive cocktail ever made (£32,500, if you’re feeling flush).

There’s history, sure. But there’s also something new: a fresh-off-the-shaker cocktail menu, filled with classic serves done the Donovan way.

Swish, glam, decadent, scintillating: adjectives you could use to describe any number of hotel bars in London, and yet somehow they feel newly minted here, in a room splashed with Terence Donovan shots and licensed until 1am Thur-Sat. The bar sits adjacent to Brown’s proper – it even has its own entrance – and is close enough that you can hear live jazz spilling in from the hotel lobby.

But where Brown’s is classic, in a “rippling piano music and wood-panelled walls” way, Donovan is cool – helped along by the photographs papering the walls, a backlit bar filled with glamorous guests grabbing a quick martini before dinner, and a joyously creative cocktail menu.

In addition to its existing list of vintage (read: expensive) cocktails, the bar has just launched Our Way, a new menu that’s refreshingly simple in concept: select a cocktail you already know and love, then prepare to meet the cooler, Donovan-ified version of it. You get all this from the collage-style menu, which arrives looking like a zine and filled with illustrations, handwritten notes and snaps of staff looking jolly. It tells you, before you’ve taken a sip, exactly what kind of bar this is.

I’m a margarita girl, and my friend – an artist who used to live in Italy – is, of course, a negroni devotee. We get stuck in, with a margarita blush (a rosy, smoky thing, served over ice) and a dolce negroni, sweet and chocolatey, like a negroni you’d get at Willy Wonka’s factory.

Fuelled by bar snacks from Michelin-starred chef Nieves Barragán Mohacho – order the confit artichoke, if you only have one – we plough on. Next for me: be(E) champagne, a honey-laced, floral mix with a hit of elderflower and a dash of absinthe, with fizz added tableside for max effervescence. It is the kind of sweet cocktail that requires a modicum of self-control, lest you suddenly find yourself looking up from an empty glass wondering where it went.

My pal goes for a pecan old fashioned – sweeter and nuttier than your gran after a few sherries, grounded with a strong whisky base note. It disappears gradually as we sip and chat our way through what must be the bar’s entire stock of olives, plus a bowl of seafood rice by Nieves.

Hours pass, or it could be minutes. Comfy chairs, fluid service, endless snacks and high-calibre cocktails transform an evening into a montage; a film trailer to look back on fondly when you eventually emerge, blinking, on to Albemarle Street. We don’t want to leave. And looking at the other tables, neither does anyone else.