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The legendary American sweet tooth may be growing up. It's classier now; more sophisticated. It lusts after salt with its sweets.

Driven by an increasingly adventurous population of palates, now even mainstream retailers and restaurants are expanding their salty-sweet repertoire. Just 0.4 percent of U.S. restaurants offered salted caramel desserts on menus in 2010, according to food and beverage consultancy CCD Innovation. This year, 3.1 percent of them do.

That four-year growth "is huge, when you think about the total number of restaurants in the U.S.," says Christine Keller, director of CCD's trend practice. "And it's still growing strong."

The strength is reflected in a mix of high-brow and lower-brow options.

Jael Rattigan, co-owner of the elegant Asheville, N.C.-based French Broad Chocolate Lounge, says some of the shop's most popular items are salted honey caramels and maple smoked-salt truffles.

"It's on people's minds now, and they're seeking it out more than ever," she says.

You could say the trend taps into our prehistoric cravings, when sweet flavors signaled nutrients and salt was a precious resource crucial to the body's balance of fluids.

"We tend to like both of those things," says Dr. Debra Zellner, visiting scientist at the Monell Chemical Senses Center, a Philadelphia institute that researches taste and smell. "Not that anybody in the United States is currently deprived of salt — but we do seek it out." In other words, It's biological.

And putting salt and sweet together seems like a no-brainer. The more modern eater — adults, anyway — will push aside a too-sweet dessert, according to Katrina Markoff, CEO of Chicago's Vosges Haut-Chocolat.

"When you add salt, it creates a cycle of continuing craving," Markoff says. "It makes your brain engage more; you're curious. The experience is euphoric and quite addicting."

Vosges sells a Pink Himalayan Sea Salt chocolate bar and pioneered the surprisingly successful marriage between cured pork and dessert in 2010 with its Mo's Bacon Bar line.

"Everyone thought it was crazy," Markoff says. "But it became something they bought by the dozen and gave as gifts."

Top toque Thomas Keller recently tinkered with Bouchon Bakery's macaron, swapping the classic caramel filling for a salted butter caramel originally developed for his New York restaurant Per Se.

But the trend is making its way down the food chain into fast food (Arby's salted caramel milkshake) and supermarket freezers (Edy's Salted Caramel Pretzel ice cream). And it has infiltrated the legion of molten chocolate cakes on chain-restaurant menus.

TGI Friday's rolled out a salted caramel cake in 2012, and while its classic brownie sundae still sells best, the new dessert has a "devoted and niche following," according to Dan Dillon, senior director of menu development.

"We were ahead of the curve when we introduced it," he says. "People want to experiment, and desserts provide that low cost of entry."

For now, salty desserts "are still a novelty, and people do pay attention to novelty," Zellner says.

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