The grass jelly I have always purchased comes in 20 oz. cans. Upon opening the can, the jelly appears black, but is actually translucent dark brown. It is also available in the refrigerator section of Asian groceries, in plastic tubs. These come with a packet of ginger syrup, and are often labeled ''Herbal Jelly.''
Grass jelly has a slightly bitter grassy flavor reminiscent of tea. It is not sweet, and has a similar clean aftertaste like good tea, largely because of the bitterness, I think. Its texture is 'brittle' like agar instead of gummy like gelatin. Like canned cranberry jelly, it takes the shape of whatever it was formed in and needs to be cut into smaller pieces for serving. Unlike canned cranberry jelly, there is a small amount of liquid in the can or container, so be careful not to spill it and stain your clothes black.
Cutting grass jelly into serving pieces is as easy as running a knife through the can or container a few times. Then pour it into a bowl, and then run the knife through it a few more times. Uniformity is overrated, just make sure that there are no extremely large pieces. Go for a range of bite sized.
My family serves grass jelly as a dessert or afternoon snack in hot weather. It is very refreshing, and can be extremely easy. We generally mix 2 or 3 cans or containers of cut up grass jelly with one can of lychees in juice or light syrup and a little bit of ginger flavored sugar syrup (to taste). We then add about a can's worth of ice cubes to the room temperature concoction, and either chill it or serve right away. Either way, the ice melts and blends with the assorted juices to create a light and tasty 'soup.'
Ginger syrup can be made by heating half a cup of sugar with one cup of water. Add as much thinly sliced ginger as you like. I recommend at least 2 teaspoonsful. Simmer until all the sugar has melted, and continue to cook for at least 5 minutes. The longer you let the ginger steep, the stronger the syrup. You will not need to use all the syrup! Avoid oversweetening the jelly or you'll end up adding more ice to water it down. Store the syrup in the refrigerator with the ginger in it, and use it for other things, like doufu nao.*
We use canned lychees as fresh are difficult to get, have a short season, and are expensive. Fresh would, of course, be infinitely better despite the extra work. I have also seen and tasted versions with other fruits added, even the ubiquitous fruit cocktail. I prefer the simple version, myself, although canned longans are pretty good too. Canned palm seeds are a bit rich, but can be tasty too.
Note that there are numerous canned snacks in Asian food markets that have grass jelly already in a sweetened syrup. While perhaps fine if you need a grass jelly fix, these are not suitable for this dessert. The pieces are invariably tiny and the syrup too sweet. Check the label, and make sure there's no sugar added before purchasing.
* I've probably mangled this transliteration, and I don't know what it is called in English. It is a very, very soft tofu that has the consistency of an extremely tender egg custard. It is plain, and is usually served hot with a little ginger syrup. It usually has a watery broth which accompanies it, since all soft tofus are mostly water and this one is no different. Fake versions, if you can believe it, also exist. These are made with gelatin, and while not inedible, are lacking in the texture area. They also dissolve upon heating, which rather gives the whole imposture away methinks!