When software engineer Gautier Coiffard, 34, told his mother he was quitting his job to open a bakery in Brooklyn, New York, she said, "That's the last thing we need ... another baker from France."

That was before L'Appartement 4F, the bakery he owns with his wife Ashley Coiffard, became an instant smash hit. The bakery opened its storefront in May, immediately turned a monthly profit and now brings in up to $128,000 per month in revenue, according to documents reviewed by CNBC Make It.

"We had a line out the door from the moment we opened, for hours and hours, until the very last croissant was sold," Ashley, 33, tells CNBC Make It.

The business is named after its humble roots. In early 2020, Gautier — a native of Grenoble, in southeast France — was craving authentic French croissants, so he started making the pastries for himself and the couple's friends.

At the time, both Gautier and Ashley worked traditional 9-to-5 jobs: He brought in $105,000 per year as a software engineer, and she was working at a mortgage company while studying for her nursing degree.

Quitting those jobs earlier this year to launch the company wasn't effortless. Today, Ashley still works as a school nurse while also managing the bakery's marketing, social media and collaborations. Here's how she and Gautier make their profitable bakery work:

A location fit for French pastries

Gautier's first croissants didn't exactly taste like a French delicacy.

"The first croissant that he ever handed me tasted like a Pillsbury Doughboy croissant, nothing special," Ashley says. "It was dense and thick ... definitely not as light and airy as it is now."

With trial and error, the croissants gradually began looking and tasting better. Gautier says he perfected his recipe by February 2020, but when the Covid-19 pandemic hit weeks later, the couple decided to wait before selling the croissants.

In June 2020, they posted their first menu on Facebook and Instagram. Friends and followers were immediately interested, but the income was modest: $150 per week, Ashley says.

Yet in the ensuing months, word of the pastries spread. The business began to take over their home.

"Slowly, the apartment started becoming a bakery," Ashley says. "We had flour covering everything we had. We had to get a storage unit so we could put our own stuff into a storage unit ... because we needed to store baked goods and flour and sugar."  

In April 2021, the couple made $10,000. Wanting to expand, they signed a 10-year lease in June 2021 for a storefront in the city's Brooklyn Heights neighborhood. A Kickstarter campaign netted them $62,000 in funding to furnish the store. They also took out $49,000 in bank loans, borrowed $72,000 from neighbors and put extra expenses on their credit cards.

Today, the storefront features floor to ceiling windows, a chandelier and vintage furniture — as any French patisserie should.

French butter and other expenses

Gautier attests that the difference between his first and current croissants is practice. French ingredients help, too.

In fact, they're among the business's largest expenses: The couple spends roughly $37,000 per month on ingredients and other food-related costs. French butter costs $20 plus shipping costs for one two-pound sheet, compared to $15 for American butter.

"The butter and the chocolate are coming from France, and are definitely more expensive than if we would buy from here ... but I really think it adds to the taste, and it should not be changed," Gautier says.

Rent adds up, too: Retail space in Brooklyn averages out to $3,200 per month, according to rental listing website Storefront. Between rent and payroll for their 20 employees, the couple pays $43,423 in fixed monthly expenses, plus an additional $3,539 in monthly credit card fees.

"We were very naive to what it actually takes to open a business," Ashley says. "At the time, we thought we were crazy and delusional — but I think we needed to be delusional to start our own bakery."

Yet L'Appartement 4F remains profitable. In June, the bakery's highest earning month so far, the couple took home $43,958, frequently selling out its 1,500 to 2,000 daily baked goods. The couple wants to open a second location — but only when the time is right.

"We want to grow very slowly and organically," Ashley says. "We want to make sure everything we do is sustainable and servicing the community that built us."

Sign up now: Get smarter about your money and career with our weekly newsletter

Don't miss:

A cardiologist shares the 5 foods she eats to lower cholesterol—and keep her ‘heart healthy’

31-year-old who quit her job and makes $15,000 a month recording voiceovers: I only work ‘3 to 5 hours per day’