One thing I like and miss about Asia is that you can always find interesting drinks. I'm not saying we don't have good drinks in Europe, yes, especially in Belgium we have tasty chocolate, maybe better coffee. But let's be honest, you find more healthy drinks in Asia and a lot of them are low in calorie so you don't have to worry too much about your weight. ^^
The café is called Gemini, located in Hanoi's Japan town or at least Japanese district, if there's no such thing as Japan town there. The café has nothing to do with Japan of course, but there's quite a few Japanese restaurants in that neighbourhood and it is one of my favorites areas in Hanoi. (I actually liked most areas in Hanoi, just love that city!)
It was a rather small place, couches are a bit small and aren't the most comfy, but it was nice, feel quite Vietnam.
They have a large selection of fruit teas, I tried the mango one and it was tasty.