This recipe for kao niow mamuang
is from Kasma Loha-unchit Clark, and appears in her book "It Rain Fishes." It is the most precise and, of course, delicious recipe of the dessert that I use.
A few notes: Use good quality coconut milk. In the US, Chaokah and Mae Ploy are the best. Of course, the best ripe, non fibrous mangoes are key. I use "Manilla" mangoes, since Thai mangoes are not available here. My wife doesn't believe me that Thai mangoes are really that good, but that's another story.
With regards to taste, I definitely recommend using pandanus leaves, if possible. It provides a subtle vanilla, floral flavor that's important in Thai desserts. Also, as Kasma notes, there should be a pronounced saltiness behind the sweetness. This is necessary to bring out the sweetness in the mangos and sticky rice.