After trying this at the only Thai restaurant in Manhattan that makes it, I never ordered the ubiquitous iced tea from them again. Actually, I never ordered anything else from them again. I don't know how they make it and I've never actually looked up the recipe, but, my version tastes almost like the real thing (I'm sure they use Thai basil, and not the common basil variety most of us are used to). I blend this in my Vitamix, but I'm sure this can also be steeped or brewed in the sun.
handful of basil (about 1/4 cup)
juice of 4 lemons
32 ounces of water
sweetener of your choice, to taste
Add all ingredients except ice to blender and blend on high for about 60 seconds. Strain, if desired. Pour over ice and serve.