To make the filling, first peel the sweet potatoes. Dice the sweet potatoes into large, 3-inch chunks. Place potato pieces in a large pot and cover with cool water. Place over medium high heat and bring to a boil. Reduce heat to medium and simmer until the potatoes are tender throughout, about 20 minutes. Test the doneness of the potatoes using a thin knife. If the knife meets any resistance, simmer the potatoes a bit longer.