For something to go down a treat it goes over very well with an audience, or has a good outcome. It is used most outside of the United States and can be used to describe food or anything really. The first term conjugates in this phrase to make went down a treat, goes down a treat, going down a treat, etc. The word treat is always singular.
A variation is for something to work a treat, which means it does its intended function very well.
Examples
We are grateful that she has forked out for the baby’s blanket/gonk needs, and so won’t bother to question why she has failed to give me the boxset of classic episodes of The Banana Splits, which I had specifically hinted would go down a treat. [The Independent]
Something that throws him back to his childhood when he hankered after something that was too much or far too expensive for his parents to buy.If he’s into gadgets, for instance, something like a Scalextric set or a remote-control car, helicopter or airplane will go down a treat. [Irish Examiner]
The 15 minute neck massages went down a treat. [Sydney Morning Herald]
My brother-in-law then puts tinfoil over the bird, held down with a heavy brick or rock, which works a treat. [New Zealand Herald]
Both young, rich, successful models who found fame at a young age – Kendall through her family’s reality show ‘Keeping Up with The Kardashians’, Cara through her meteoric rise to modelling stardom at age 17 – with huge fanbases (Cara has 8.4 million Instagram followers, Kendall has 16.5 million) this is a friendship their respective publicists could only dream of – and it looks like it’s working a treat. [The Telegraph]