Sương Sáo aka Grass Jelly is a dessert/drink popular throughout Southeast Asia. According to Chinese medicine it has "cooling" properties and is good for the skin. Grass Jelly is actually made from a dried herb that is a member of the mint family...not grass as the name suggests. The herb's name in Chinese which is "len fan chou" (cooling grass). Thus, jelly made from cooling grass is....you guessed it Grass Jelly.
You can purchase Grass Jelly at just about any Asian grocery store for dirt cheap...but where's the fun in that?
"Grass" to make Grass Jelly can be found at most Chinese Herbal/Medicine Shops. Rinse/wash the grass, add water to cover the grass by 2", simmer for at least 1 hour. 
Cool and strain the grass through a cheese cloth or a fine sieve or...both. The liquid obtained will be used as the base to make grass jelly.
To make grass jelly.....
Use a measuring cup to measure the liquid obtained after straining out the herb. Dilute that liquid with an equal amount of water. For example, if you got 2 cups of liquid add 2 cups water (+/- depending on how pronounced you want you jelly to taste). For every 6 cups of diluted liquid you will need 75g of tapioca starch (+/- depending on how firm you want you jelly to be).
Bring 5 cups of liquid to a boil. Meanwhile, dissolve the tapioca starch in the remaining 1 cup of liquid. As soon as the liquid in the pot begins to boil, stir in the starch mixture. You'll notice the liquid will thicken immediately. Mix well and fast, then pour the mixture into a clean bowl and allow to set. The jelly set right before you eyes...literally. In about 20 mins the grass jelly is ready to be served.
To serve: cut the jelly into small pieces, mix with sugar or honey...grass jelly also pairs exceptionally well with Nước Rau Má (Pennywort drink)....I was going to post a recipe but my dear friend Lily beat me to it...you can find her recipe here
http://lilyng2000.blogspot.com/2010/07/pennywortpegaga-drink-secret-fountain.html Just in case you're wondering...this isn't 'real' grass jelly since the starch is setting the jelly and not the grass. I thought so too...so I experimented. I tried to thicken 6 cups of water with 75g of tapioca starch to see if the liquid would set like grass jelly....it didn't...as you can see from the picture below. The white mess in the bowl is the water + tapioca starch vs. grass jelly + tapioca starch.
The starch triggers a chemical reaction with the grass jelly liquid to set the jelly. The starch alone can not do the job. Besides tapioca starch you can also use corn starch, mung bean starch, arrow root starch or rice flour. However, I found that tapioca starch gives the jelly the best appearance (clear and pitch black). If you prefer a very soft jelly, use 30g of starch (it's the minimum amount of starch you'll need to set 6 cups of grass jelly; the more starch the firmer the jelly). For a slightly chewy texture add 1/2 tsp
lye water.
