As birthday parties go, Barbara Maier Gustern’s won’t be your typical candle-blower. The singing teacher turns 85 on Monday, a milestone she’s celebrating with a concert starring two dozen of her students — Debbie Harry, Heidi Rodewald and Stew among them.

And every penny from the $30 and $250 tickets for the Joe’s Pub bash will benefit Gustern’s favorite soup kitchen. “I wanted to do something good for the world, and I know the work they do,” says the tiny (4-foot-10, 85-pound) philanthropist, who’s lived across the street from Chelsea’s Holy Apostles Soup Kitchen for 45 years. “I’ve had some bad things happen to me, but I’ve had a wonderful life.”

Though she’s sung and taught any number of operatic arias, most of her students — Taylor Mac, Tammy Faye Starlite, Penny Arcade, Justin Vivian Bond, Martha Graham Cracker — skew decidedly more downtown. She herself grew up in Boonville, Indiana, where, as a little girl, she’d hop up on a chair and sing the World War II songs she heard on the radio.

Nevertheless, her parents didn’t consider singing a practical career choice. She persuaded them to let her come to New York to pursue a psychology degree at Columbia University. She was closing in on a Ph.D. when she decided the hell with her dissertation: She wanted to perform.

“In my day, if you were a singer auditioning for Broadway and the opera, you paid your rent by having a church job and a temple job,” she says. Her Jewish-sounding maiden name helped get her an audition at a synagogue. Though the rabbi hired her, he wondered why she knew no Hebrew. “I told him I came from a mixed marriage,” she says, telling herself it was true, since her father was a man and her mother, a woman.

It was at the temple that she met her future husband, also a singer. “The Torah reading was so long, we went out for coffee, talked and hit it off,” she recalls of that first meeting with Joe Gustern. But as his career took off, hers stalled. She was miserable, until a voice teacher she met at a party told her the American Musical and Dramatic Academy was hiring. Though Gustern never taught a soul in her life, she went.

“They gave me a student and asked me to do a demo [lesson],” she recalls. “I thought, ‘I love this!’” She booked the job.

Her client list took a turn in the mid-’80s, when she began working with singer and performance artist Diamanda Galas, then struggling with vocal issues. Soon after, Blondie’s Debbie Harry, a big Galas fan, came calling. So did most of downtown’s cabaret performers and, decades later, the cast of Broadway’s 2019 “Oklahoma!”

“She had such joy coaching us,” says Mary Testa, who’ll perform, along with several other “Oklahoma!” cast members, at Monday’s benefit. “She kept saying, ‘I’m so happy to be here!’ ”

Though Gustern delights in her work, she’s no stranger to grief. Her only child died in 2003, leaving Gustern with a grandson and a need to honor the daughter she lost. Since her daughter loved dogs, Gustern threw a benefit, raising $4,000 for an animal-rescue organization. Other fundraisers followed. The last — in honor of her husband, who died in 2017 — aided the Actors Fund.

“I’m very, very fortunate,” says Gustern, who teaches 20 hours a week and starts her day stretching in a bathtub full of hot water. “I’ve got enough money and tons of friends. When I’m teaching these people, I haven’t the foggiest notion we’re not the same age.”

Ref;nypost.com