1) In a small bowl, mix together chopped coriander, olive oil, minced garlic, 1/4 tsp of the salt and 1/8 tsp of the pepper; set aside.
2) In medium bowl, mix together brown sugar, mustard, hoisin sauce and vinegar. Reserve 2/3 of this mixture. To remainder, add 120ml lime juice and stir in chicken; cover and refrigerate.
3) Place sweet potatoes in a heavy saucepan and cover with boiling water. Cook, covered, over medium-high heat until tender, about 15 minutes. Reserve 60ml cooking liquid, then drain potatoes in colander. Return potatoes to reserved cooking water and add butter, chipotle pepper, adobo sauce, remaining 5ml lime juice, cumin, lime zest, remaining 1/4 tsp salt and remaining 1/8 tsp pepper. Mash potatoes.
4) Preheat the grill. Thread chicken on 8 bamboo skewers that have been soaked in water for 30 minutes. Broil about 15 cm from heat, basting with reserved mahogany sauce until done, about 8 minutes. To serve, divide potatoes among 4 plates; top each with 2 skewers of chicken and drizzle with coriander chimichurri sauce. Garnish with coriander sprigs.