Most North American barbecue sauces play the sweetness of sugar or molasses against the sharpness of vinegar and hot sauce. A similar contrast of sweet and sour characterizes the barbecue sauces of Thailand. The following sauce, inspired by one served at the restaurant Bahn Thai in Bangkok, derives its piquant flavor from tamarind. It is particularly good with grilled chicken or fish. I leave the number of chiles you use up to you.