Remove the grassy tops of the lemon grass, leaving stalks about 6 inches long, and cut off any hard root section. Using the blunt edge of a large knife or cleaver, bruise each stalk all over, then chop. Combine marinade ingredients in blender, process until smooth and pour into a nonreactive bowl. Add chicken, turning to coat all over. Cover and refrigerate at least 4 hours or overnight.

Meanwhile, combine the sauce ingredients in a small saucepan and bring to a boil while stirring. Reduce heat and simmer 2 to 3 minutes, or until thickened slightly, then remove from heat.

Prepare grill or preheat broiler. Remove chicken from marinade, and grill or broil breasts and thighs until golden brown on all sides over high heat, starting chicken skinside down first. Then move chicken to a medium hot part of the grill or broiler until cooked through, 20 to 30 minutes total. Serve with Sweet and Spicy Sauce.