A London coffee shop is charging $64 for its premium brew -- here's what it tastes like

With only 15 servings on offer, this isn't your average latte or flat white, so don't even think about asking for milk, sugar or a sprinkling of chocolate on top.

Based in a very upscale district of Central London, the Queens of Mayfair coffee shop has already sold more than half of its available stock, which can only be drunk on the premises as part of an exclusive experience.

Victoria Sheppard, who founded Queens of Mayfair with her sister, Grace Sheppard, told CNN that one customer, who described themselves as a "coffee fanatic" is traveling from several hours away to sample the drink.

"We have coffee connoisseurs coming to us from all over the place," she said, describing their latest acquisition as a "very rare commodity".

This particular lot of Ethiopian coffee beans won first place in the Cup of Excellence, a prestigious annual competition for high-quality coffees.

It was bought back in June and roasted last month by Difference Coffee, the roaster that supplies Queens of Mayfair and one of only eight firms worldwide that was invited to bid on the beans.

The coffee was graded 91.08/95 by the competition's international panel of judges and coffee experts following rigorous tasting and testing, according to Queens.

The beans retail for up to £2,000 per kilogram (around $2,600) and are truly fair trade as the Cup of Excellence program is designed to reward farmers for excellence, which means they receive most of the auction price -- 155 times the normal commodity price.

It's one of many rare premium coffees the sisters intend to showcase at their cafe, which opened just last month.

Aside from this flagship flair, however, the rest of the menu is considerably more accessible. A regular espresso goes for just over two pounds and you can pick up British classics like a sausage roll for £4.50 or cheese and toast for £6.

The cafe, which also serves reasonably priced wine and cocktails, opens late to catch the evening crossover crowd.

CNN Travel headed down to Mayfair, where head bartender Massimo Golfetto weighs out the portion to the gram, grinds it by hand at our table and expertly brews it in a glass V60 filter, which has a 60-degree angle, allowing the coffee to drip at an optimal flow rate.

"It's lighter, almost see-through," he explains, as he pours the brew into the curved glass with its elegant stem. "It looks more like a tea than a coffee."

As an Italian used to punchy espressos, he says, "When I had the coffee for the first time, it was a completely different experience."

Raising the glass and taking a sip, the aroma is floral and the flavor delicate and light. It may be one of the few experiences of 2020 not to leave a bitter taste in the mouth. When it comes to coffee, this creation proves that might isn't always right.

The curved glass "catches all the flavors, to get a better experience on the nose," explains Golfetto.

The flavor evolves as the coffee cools, its character unfolding like petals.

"It's like a fine bottle of wine," says Sheppard. "It's not about the strength or the most pungent coffee. It's about the flavor complexity and interesting tasting notes."

The process involves a three-minute "blooming phase," which is when the flavor really develops and intensifies, she explained.

In all, 250 grams of water is added to 15 grams of coffee. The liquid passes through the glass filter into a carafe, before being served in a crystal glass.

Purists will be pleased to hear that the coffee is served black and straight up.

Adding milk would be "like getting a fine wine and topping it up with soda water," says Sheppard. "It's very much about the raw taste in something organic and unmodified."