For the Chicken: Preheat oven to 400 degrees F. Combine the spices in a medium bowl. Brush each poussin with oil and rub the spice mixture over the entire bird. In a hot large saute pan sear all sides of poussin, until golden. Place the birds on a rack over a baking sheet and roast the birds for 25 to 30 minutes, until just cooked through. Let rest 10 minutes before serving.
For the Mango Sauce: Place stock in a medium saucepan over high heat and reduce to 4 cups. Heat oil in a medium saucepan over medium heat, add the onions and cook until soft. Add the roasted garlic and cook for 1 minute. Add the mango, and cook for 2 minutes. Add the reduced stock and brown sugar and cook slowly over medium heat until the mango is very soft, and falling apart, 30 to 40 minutes. Place the mixture, in batches, in a blender and blend until smooth. Strain the mixture into a clean saucepan, whisk in the roasted garlic and cook for 10 to15 minutes until reduced and slightly thickened. Stir in the cilantro and season with salt and pepper to taste.
For the Plantains: Heat oil in a cast iron skillet to 365 degrees F. Fry the plantains in batches until golden brown. Remove them with a slotted spoon to a plate lined with paper towels and season with salt to taste.