When the butler walked into the dining room at the Istana last Monday with a birthday cake for Mr Donald Trump, a wide grin broke out on the American President's face.

The US leader and his delegation were having a working lunch with Prime Minister Lee Hsien Loong and other Singapore officials ahead of his summit with North Korean leader Kim Jong Un when the surprise was sprung on him.

But the 3kg chocolate cake topped with a golden candle was no surprise at all for those who had been planning the lunch for two weeks.

It is what Ministry of Foreign Affairs protocol officer Jessica Tan describes as an "engineered" surprise, and one of the ways a host country adds a personal touch to a visiting leader's trip.

Ms Tan had done thorough research on Mr Trump's food preferences before settling on a chocolate cake. The US President has previously waxed lyrical about the chocolate cake at his Mar-a-Lago club in Florida.

The early birthday celebration for Mr Trump - the working lunch was three days shy of his birthday - highlights how nothing was left to chance in planning for the visits of the two foreign leaders and the summit between them.

Behind the scenes, Ms Tan had been working with her US counterparts on the meal, planning and scripting everything down to the smallest detail.

Ingredients were discussed and portions were carefully calibrated to make sure there would be enough and yet have no wastage. Even the timings between the serving of the different courses was rehearsed.

"Essentially everything that is done is meant to facilitate having a conversation, to allow PM Lee and President Trump to have a good conversation in a very informal setting," says Ms Tan.

"Even though it was short and our role was just confined to the Istana supporting PM Lee in these bilateral elements, but just the weight of it and knowing (the summit is) taking place in Singapore, it's quite a privilege."