Preheat the oven to 425 degrees. Line a baking sheet with foil. Pierce the sweet potatoes in several places with the tip of a paring knife. Place on the baking sheet and bake 40 to 50 minutes, until soft and oozing. Remove from the oven and when cool enough to handle, peel and place in a food processor fitted with the steel blade (you can also blend the mixture with a hand blender). Turn the oven down to 350 degrees. If not using a crust, butter a round 9-inch baking dish.Measure out 1 1/2 cups of sweet potato purée (store any extra purée in the refrigerator and enjoy with a meal). Place the purée in the food processor, or in a bowl if using a hand blender, add the remaining ingredients (except the crust) and blend until smooth and creamy. Scrape into the baking dish or the crust. Place on a baking sheet and bake for 45 to 50 minutes, or until set. Remove from the oven and allow to cool completely.