Salted egg yolk custard croissants are the first food craze of the year in Singapore.

The croissants, which originated in Hong Kong, started making inroads here late last month, when brunch cafe Flavour Flings in Hougang started selling them.

Since then, five bakeries and cafes have jumped on the bandwagon to offer the sweet-savoury pastry.

The latest to fuel the craze is Italian deli chain Da Paolo Gastronomia, which will sell its salted egg yolk croissants from today.

It joins French patisserie chain Antoinette, Kokomama Marketplace in the School of the Arts, Bridge restaurant-bar in Seah Street and Black & Ink in Changi Road, all of which started selling the item earlier this month.

Do.Main Bakery in Tanjong Katong Road will launch its version on Saturday.

It was European bakery chain Urban Bakery in Hong Kong that started offering molten egg yolk croissants in September 2014.

The move followed the success of its Ball Law Cowssant, a croissant topped with the crumbly cookie dough found in bo lo bao (pineapple buns). The chain sells more than 1,000 molten egg yolk croissants across its 10 outlets daily. Each croissant costs HK$22 (S$4).

Mr Rico Cheng, 30, food concept manager of Maxim's Group, which owns Urban Bakery, says he was inspired by the popular dim sum item liu sha bao - steamed buns filled with flowing salted egg yolk custard. "We are thrilled and proud that the croissant,which was created to celebrate Hong Kong's food culture, has become a hit in other countries too," he says.

He adds that more tourists, mostly from Singapore and Malaysia, have been buying the pastry. The craze has also spread to Kuala Lumpur and Johor Baru in Malaysia, and Jakarta, Indonesia.

Some home-grown eateries have been quick to spot a potential hit.

Antoinette, Da Paolo Gastronomia and Kokomama Marketplace make their croissants from scratch, while the other cafes get theirs from bakeries or pastry suppliers. Most of them pipe the filling into the pastry from a hole at the top or base of the croissant.

Da Paolo Gastronomia director Francesca Scarpa, 33, says a love for liu sha bao and requests from customers prompted her to expand the chain's croissant selection.

She says: "We use the dim sum favourite as a reference to create the taste and consistency of the custard filling, which has a touch of coconut milk."

The chain has been nimble in adapting to food trends. It introduced its version of cronuts, a hybrid of croissants and doughnuts, in 2013, when the confection was trending in New York City.

Ms Scarpa says: "We can react fast to such trends as we can tap on our croissant-making expertise and experiment with new recipes."

At Antoinette, sales of its liu sha croissants have doubled over the past three weeks, with 600 sold within four hours daily across its three outlets. Its chef-owner, Mr Pang Kok Keong, says the craze has sparked interest in his pastries.

"The pick-up rate of these pastries has not been as good as our cakes, but more people are interested to understand what makes a good croissant," he says.

The chain also enjoys a spillover effect on its dine-in business, as diners order the croissants to go with their meals there.

For cafes such as Kokomama Marketplace and Black & Ink, getting in on the custard croissant trend is a way to extend their existing selection of salted egg yolk dishes, such as pizza, pasta and muffins.

Kokomama director Chris Chew, 54, says: "We have been developing croissants stuffed with red beans and matcha since November, and when the trend hit Singapore, we didn't want to be left behind."

While there has been an increase in walk-in sales, he says business has not risen significantly.

Mr Lawrence Ngan, 35, managing director of Bridge, which sells up to 250 croissants a day filled with salted egg yolk or truffle honey custard, says: "People from the offices nearby have been ordering up to 20 pieces at a go, but we do not sell them during dinner time as diners will be full after eating one and it makes business sense to focus on dinner service."

As salted egg yolk custard croissants gain popularity, health experts caution against indulging in them too often.

Ms Claudia Correia, a dietitian at Raffles Diabetes & Endocrine Centre, says a plain croissant is rich in saturated fat and salt, and when salted egg yolks and custard are added, the pastry can contain about 600 calories and up to 80 per cent of a person's daily recommended intake for saturated fats.

She says: "I recommend sharing the croissant and eating it mindfully by paying attention to your senses and body cues to reduce overeating."

Ms Vanessa McNamara of The Travelling Dietitian says: "Those with coronary heart disease or are trying to lose weight should minimise their intake of salted egg yolk croissants."

However, diners are still drawn to the pastry.

Digital strategist Rebecca Saw, 35, who is Malaysian and based in Kuala Lumpur, came here to try the croissants from Antoinette, after her Singapore friends told her about them.

She says: "They are my second favourite after Urban Bakery's as they have a crisp and good pastry. However, the filling could be sweeter and it lacks the full-bodied flavour of a liu sha bao filling."

Project manager Michelle Nah, 38, who has tried the croissants from Antoinette, Bridge and Kokomama Marketplace, was curious about the social media craze over them.

She says: "I like Antoinette's croissants best as the custard has the most intense salted egg yolk flavour and oozes like nobody's business.

"But I think that croissants and salted egg yolks make a weird pairing. I still prefer croissants with sweet fillings such as chocolate."