A midtown ad executive plunged to her death when her balcony gave way while she was having a cigarette and chatting with a man on their first date early Thursday, police and reports said.

Jennifer Rosoff, 35, fell from the 17th floor at 400 E. 57th St. and landed on scaffolding on the first floor.

"I heard a bang. I thought it had been a gunshot," soda executive Steve Hersch, who lives in the building, told Dnainfo.com.

The city medical examiner ruled Rosoff's death, which occurred around 12:50 a.m., an accident.

The Department of Buildings ordered all the prewar building's balconies off-limits until an engineering report is filed and further inspections are conducted.

"There appear to be no open violations on the building," but it was not yet clear if the building was up to date on its inspections, said Department of Buildings spokeswoman Kelly Magee.

Rosoff, who lived on Long Island before moving to Manhattan, was remembered yesterday as a generous, talented and driven professional who made friends wherever she went.

"She was just unbelievably helpful," said Danny Bellish, 26, a sales manager for Saveur, a food magazine.

Rosoff took Bellish under her wing around 2009 when he was an assistant print buyer wanting to change careers and she worked as an account manager at Cosmopolitan.

She literally drew him a map of everything he needed to learn in digital advertising to help him succeed, he recalled. "I still have it," he said of the instructions she drew for him on the back of a menu.

Also, "she got my girlfriend an internship," and even arranged a trunk show at Hearst to help his mother's jewelry business, Bellish said. "I turned to Jennifer at every single step of my career," said Bellish, adding "she was always ahead of the curve," and the first to spot industry trends.

Rosoff had been working as sales manager at TripleLift, a Fifth Avenue advertising agency, since April.

"The entire company is distraught," TripleLift CEO Eric Berry said in a statement, noting that Rosoff's "tremendous energy and humor brought so much joy to the office."

Rosoff graduated from Walt Whitman High School in Huntington in 1996 and earned a degree in communications from Tulane University.

According to Rosoff's profile on networking website LinkedIn, she had worked at The New Yorker, Lucky, Cosmopolitan and In Touch magazines, as well as at Getty Images. "She was a very wonderful woman -- really positive and great to be around. She lit up a room," said another former colleague who requested anonymity.

Rosoff was a smoker, Bellish confirmed, "but she had been trying to quit for some time."