In a small food processor or a blender, combine the cilantro, garlic, pepper, soy sauce, fish sauce, salt, and water. Grind to a fine, fairly smooth paste. Add another tablespoon or two of water if needed to help grind evenly.
You can also mince the garlic and cilantro finely, mash them with the salt on your cutting board, and scrape the mixture into a bowl to mix with the pepper, fish sauce, and soy sauce.
Transfer the paste to a large, deep mixing bowl and add the chicken pieces, turning to coat everything well. Cover and refrigerate at least an hour or even overnight, turning occasionally to coat evenly with the marinade.
Prepare a very hot fire in a charcoal grill, or heat a gas grill or oven to 450 degrees F. When hot, place the chicken on the lightly oiled rack, or in a roasting pan in the oven, and cook, turning occasionally to brown and cook pieces evenly, until chicken is cooked through. Serve hot, warm, or at room temperature, with small bowls of Sweet-Hot Garlic Sauce.
For the Sweet-Hot Garlic Sauce:
In a medium saucepan, bring the sugar, vinegar, water, garlic, and salt to a gentle boil over medium heat, stirring well to dissolve the sugar.
Simmer 8 to 10 minutes, until you have a thin, smooth syrup. Remove from heat, stir in chili-garlic sauce, and set aside to cool. (If you use fresh hot red chili peppers or dried red chili flakes, use a fork to mash them up on your cutting board with the garlic and the salt, and then scoop the coarse paste into the warm sauce.)
Serve at room temperature with Grilled Garlic Chicken, Issahn Style , or any other grilled or crisp-fried food.
Sealed in a glass jar, it keeps in the refrigerator for up to 1 week.