Preheat heat oven to 425 degrees. Place chopped potatoes and ½ cup of water into a 9-by-13-inch baking dish, covered with foil. Bake for 1 hour covered and 15 minutes uncovered. Remove dish from oven and add remaining half cup of water to loosen up any bits in the baking dish.
Place all the solids and liquids into a strainer lined with cheesecloth. Let drain into a saucepan and cool for 30 minutes. Once cool enough to handle, squeeze as much liquid out from the potatoes in cheesecloth as possible. You should end up with a cup and a half of liquid.
In a saucepan, bring the sweet-potato liquid to a boil, then reduce heat for a steady simmer. Allow the liquid to reduce for 15–20 minutes until it starts to thicken and starts to form a caramel; stir often in the final minutes. Pour into a jar and in store the refrigerator for up to 2 weeks.
Can be served with roast meats or as a sauce over ice cream or brownies. Oh, and don’t forget to eat the leftover sweet potato mash!