Growing up, my sister and I referred to this as "The Eyeball Drink," served by Mom as an afternoon refresher on a hot day. But really, it was nothing more than basil seeds re-hydrated in water with a bit of sugar. You can literally pour a bag full of basil seeds into a glass of water and watch as they go from seeds to these translucent alien-like objects. $2.50 a cup is the going rate in Chinatown. Some places mix in sugar, others add liquid sugar, and others such as Banh Mi Sau Voi Cafe add in "pourtailai," pictured above. I don't know what the English translation is, but it's got texture of fine seaweed, thick and a bit crunchy. A fine pairing, if I do say so.
Banh Mi Sau Voi Cafe
101 Lafayette Street, New York NY 10013 (map) 212-226-8184