Adapted from "Vietnamese Home Cooking," by Charles Phan. This recipe will work without shrimp heads, but Phan says the heads add richness to the sauce. The recipe also calls for homemade roasted chile paste (there's a recipe in the book), but if you buy it look for jars labeled chile-bean or saté paste, preferably without preservatives. Phan adds that in a pinch you can make the recipe without the chile paste. Serve with steamed white rice.