Place the sweet potatoes in 4-quart saucepan, and add water to cover. Heat to boiling, then reduce heat and simmer until tender, about 12 minutes.
Drain the potatoes and return to pan. Add 1 cup of the Greek yogurt, the butter, 3 tablespoons of the brown sugar, the salt and remaining spices. Mash with potato masher until smooth.
In small bowl, mix remaining ½ cup Greek yogurt and remaining 1 tablespoon brown sugar until smooth. Serve potatoes warm, topped with sweetened yogurt and chives.