SOUTHOLD, N.Y. — “I think I finally have the proportions right,” said Claudia Fleming, the pastry chef, narrowing her eyes at the chocolate caramel tart perched on her palm. She was standing in the kitchen of her Long Island restaurant, the North Fork Table.

It was an absurd statement. Ms. Fleming, 60, has been making this tart since the 20th century.

She created it for Gramercy Tavern in Manhattan, where she was the pastry chef for nearly a decade; tinkered with it there as it became her salty-sweet signature; and published the recipe in her 2001 cookbook, “The Last Course.” A cult classic among pastry chefs, the original edition has become a precious rarity; copies that occasionally pop up on eBay sell for as much as $200. The book is finally being reissued this month, at the same time that Ms. Fleming is closing a difficult chapter that culminated in the death of her husband, the chef Gerry Hayden, in 2015.

She could make this tart with her eyes closed, and it would still be the best chocolate caramel tart you’ve ever tasted. But until it is the best chocolate caramel tart she has ever tasted, it is not finished.

There are only three elements, none revolutionary. It is a chocolate crust lined with soft caramel and filled with chocolate ganache. Any good chef would ask: Are these perfect together?

But Ms. Fleming asks: Are they perfect in relation to each other? Precisely how much salt is needed in the crust to backstop the caramel? Does it need to have corners, so that some bites are crunchier than others, or should it be round, which will make for a oozier center bite? How many millimeters of caramel best complements how many millimeters of chocolate?

Cooking skills are easy enough to learn. But to carve a path, like Ms. Fleming, a chef needs two things that cannot be taught: a great palate and a deranged sense of perfectionism.

Ms. Fleming grew up on Long Island, and as far back as she can remember, she worked toward one dream: to be a professional ballet dancer. By age 18, she knew she was never going to be tall enough or thin enough to be a ballerina, though her teachers told her it was easy to be thin: Just stop eating.

“I thought I was buying into it, but I wasn’t,” she said, referring to the punishing ethos of traditional ballet. “And my mother wouldn’t have stood for it.”

Pushing through the endless repetition of steps and the pain of dancing on point, she said, was probably good training for a restaurant kitchen. She switched to modern dance, moved to New York City and gave herself a deadline of age 25 to make it as a professional dancer. In the meantime, she supported herself by working in restaurants.

Her first job in high school was scooping ice cream at a Friendly’s at the South Shore mall.

“I loved it so much,” she said dreamily. “I love ice cream more than anything. I loved the fast pace, the organizing, the physical challenge.”

She started at Jams, which the young chef Jonathan Waxman opened in 1984 to introduce Manhattanites to the new “California cuisine.” The chefs Alice Waters, Evan Kleiman and Judy Rodgers were already running influential kitchens on the West Coast, and most of the line cooks Mr. Waxman had recruited were women. Ms. Fleming said it drew her toward the kitchen. In New York, she said, “That was literally unheard-of at the time.”

She waited tables at Union Square Cafe, where the owner Danny Meyer said he hired her before she sat down for an interview. “Even from half a room away, I could feel her warmth, poise, and intelligence,” he wrote in the preface to “The Last Course.” She moved into the kitchen there as a line cook, and was hired away by Drew Nieporent as a pastry assistant when Tribeca Grill opened in 1990.

“Once I tried pastry, that was it for me,” she said. “I was hooked.”

At the time, American pastry chefs were quite literally reaching new heights. Desserts were being reinterpreted, deconstructed and reconstructed on tall scaffoldings made of spun sugar, airy mousses and puff pastry. After decades of French standards like chocolate mousse and crème caramel, the simultaneous arrival of nouvelle cuisine and global express shipping brought new playthings to American restaurant kitchens, like white and dark chocolate, macadamia nuts and mangoes, kiwi fruit and coconut milk.

Ms. Fleming dived into exploration, but only dabbled in construction. “I wasn’t very interested in Legos,” she said. “I wanted it to taste like something.”

She gave up on dancing and pursued a new dream: to immerse herself in the tradition of French pastry in Paris. “At the time, my money would last longer there than in L.A.,” she said; her other idea was to go deep on bread at Nancy Silverton’s groundbreaking La Brea Bakery.

With help from the New York baker Maury Rubin (“I nagged him for two years to get me in”), she apprenticed at the celebrated, century-old Patisserie Rousseau et Seurre, then at the influential emporium Fauchon, learning mostly by observation.

“I was sometimes allowed to cut the dough, but never to make the dough,” she said. “Maybe they would let me dip the éclairs if someone didn’t show up for work.”

After she returned to New York, Ms. Fleming checked in with Mr. Meyer, who was about to open his luxurious, gloriously American restaurant Gramercy Tavern with the young chef Tom Colicchio in charge of the kitchen.

“It was not love at first sight: I was extremely intimidated,” she said. Mr. Colicchio had worked in pastry himself, and she had a long résumé but little hands-on experience. “I was in so far over my head.” Fortunately, she said, at first he just told her what desserts to make.

When she got her bearings, Gramercy Tavern had become a destination restaurant, and she felt ready to invent desserts that were not only as multidimensional as the rest of the menu, but also as intensely flavored and respectful of ingredients.

“I wanted to cook my desserts, like a chef, not just scoop and slice,” she said.

Eventually, her signature style of starting with a single flavor, such as apple, mango, almond, ginger; intensifying it; and finally setting it off with a bit of crunch or tang or cold, came into focus. Her techniques for panna cotta, fresh corn ice cream and roasted pears became modern classics; her use of savory ingredients like rosemary, cornmeal and black pepper helped change the language of American desserts.

Ms. Fleming published “The Last Course,” written with the New York Times columnist Melissa Clark, in 2001. The same year, she and Mr. Hayden, her former boss at Tribeca Grill, were married. In 2005, they decided to leave the city’s pace and grime and moved to the North Fork of Long Island, a region burgeoning with local wine and food, to take over — and reinvent — a small 18th-century inn.

She had left the scene, but her style seeped through high-end New York restaurants as chefs who trained with her rose to top kitchens, like Deborah Racicot of Locanda Verde, Karen DeMasco of the Jean-Georges empire and Gina DePalma, the original pastry chef at Babbo.

Ms. Fleming’s book went out of print, so young pastry chefs who wanted to learn from her had to exchange photocopies and hold onto dog-eared, caramel-splattered first editions.

Safia Osman has been working in pastry kitchens in Dubai, Hong Kong and elsewhere for a decade, but has always kept her copy with her. “Whenever I am stumped for a new dessert, wherever I am in the world, I can open that book and reread it for inspiration,” she wrote in an email.

The book is also a reminder to keep it simple. “Those desserts are beautiful and delicious of course, but they also seem natural and effortless,” she added. “That’s what I strive for as a chef.”

“The book is a legend,” said Matt Sartwell, an owner of the Manhattan cookbook store Kitchen Arts and Letters, calling the new printing a “godsend.” “So many people have read or heard something about it that lights a fire under them.”

At North Fork Table, Mr. Hayden and Ms. Fleming built networks with farmers and fishermen, and pulled together a food community around the inn’s tiny kitchen. But the work of running it, along with innkeeping, was overwhelming from the start, she said. In 2011, Mr. Hayden received a diagnosis of amyotrophic lateral sclerosis (A.L.S., or Lou Gehrig’s disease), and she cared for him until he died four years later.

Only now, she said, has she begun emerging from those dark years. She is planning to sell the inn, moving back to New York City, and searching the menus of her culinary descendants for clues to her next act in pastry. Will it include black sesame? Boba? Burned sage?

“I’ll think of something new,” she said.

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