"One of them was the muffle, a muffin and waffle. We did make one, but the fruit caramelized too quickly," Vo tells The Salt. "The trick to fusion desserts is to bring out the best of both." That's why Vo's invention — the "townie" — has been a huge success. The brownie-tartlet has a gooey center and a crisp outer shell.

"I immediately saw it and thought, 'Why hasn't anyone done that before?' " Vo tells The Salt. She says the fact that brownie dough isn't very wet prevents the shell from getting soggy and also allows for underbaking the brownie to give the center that highly sought-after gooeyness.

But while Vo knew it would work, she didn't expect to sell more than a few dozen. Instead, she sold 800 townies in 10 days and garnered attention from U.S. TV networks. "This is one of those unusual things that really had its own life," she says.

Vo was already beloved by customers for her duffin, a cake doughnut filled with jam, which she created a few years ago. Photo: Helena Marie Fletcher/Bea's of Bloomsbury

These days, food fads are accelerated by social media, which can be good and bad. "Things blow up bigger and then die faster," says Marian Berelowitz, who writes about food trends for analyst firm JWT Intelligence. "Instagramming or tweeting a picture of your cronut [or any other fad] gives you social currency and becomes a huge part of the appeal."

Berelowitz says the incessant stream of images surrounding a particular food or product can create FOMO — fear of missing out — among friends and followers. The very act of posting on social media can make the time spent queuing fly by. "Standing in line is far less tedious," she says, "when you can share the experience with your Facebook friends."

But, the deceleration can happen just as fast, Berelowitz warns. "Word gets around at lightning speed, the buzz is so much more amplified, and then attention moves on."

So how do fads like the cronut and the townie impact the small-business owners behind them? Berelowitz says the publicity can provide a long-term boost, as long as the product line isn't a one-trick pony: "It's rarely a sustainable proposition when a business revolves solely around one fad product."

New Yorkers will remember Beard Papa, the Japanese cream puff purveyor that gained a fan following starting in 2004. The fad lasted a few years, and the chain opened branches throughout the city. Now, there is only one shop in Manhattan.

But for baker Bea Vo, the townie has helped her existing product line. It has revived interest in her duffin and inspired her to produce a cookbook exclusively of hybrid dessert recipes.

"The cronut is hard to make without a commercial kitchen," she notes. "I want to write recipes people can make at home."

Copyright 2013 NPR.

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Forget Cronuts: London's 'Townies' Take on Hybrid-Dessert Craze