In a 9-by-13-inch baking dish, bake about 3 lb. sweet potatoes, peeled and chopped, with 1/2 cup water (covered with foil) in a 425°F oven for 1 hour. Remove foil and bake for 15 minutes longer. Add another 1/2 cup water and scrape up any bits stuck to baking dish. Set a strainer, lined with clean cheesecloth or a kitchen towel, over a saucepan. Pour in sweet potatoes and any liquid. Once potatoes are cool enough to handle, wring and squeeze out as much liquid as possible. (Save leftover sweet potato mash to eat.) Simmer strained liquid until it starts to thicken and form a caramel, 15 to 20 minutes, stirring often at the end. Pour into a jar (you will have 1/4 cup caramel) and store in refrigerator for up to 2 weeks. Before serving, warm up and loosen with butter or water as needed. Add salt to taste, if desired. Spoon it anywhere you would a traditional caramel, such as on your favorite cake, ice cream or chocolate desserts.