NOTE: The following mixture may be substituted for 1 teaspoon Thai chili paste: quarter ts cayenne, quarter tsp sugar and half tsp oil Wash, peel, de-vein shrimp. Save shells. Wash shrimp again, drain, pat dry, cover and refrigerate. If using fresh lemongrass, cut each stick into three 5 cm pieces--starting from rounded bottom end. Discard straw-like top. Lightly crush the 6 pieces. In a pan, combine lemongrass, lime leaves, stock, and shrimp shells. Bring to boil. Lower heat and simmer gently for 20 minutes. Strain stock, then add fish sauce, lime juice, and chili paste. Adjust fish sauce and lime juice to taste. Add more chili paste for more heat. Drain straw mushrooms and add to stock. (If using fresh mushrooms, quarter them and drop in lightly salted boiling water. Boil 1 minute. Drain and add to stock.) The soup can be prepared to this point several hours ahead of time and stored in the refrigerator.* Prepare garnish shortly before serving. Cut green chilies into fine rounds. Wash and dry cilantro. Just before serving, heat the soup, when it begins to boil, drop in peeled shrimp. Cook on medium heat for 2 minutes or just until shrimp turn opaque. Garnish with chilies and cilantro leaves. Serve hot.