In saucepan, cook sweet potatoes, liquid, and brown sugar over low heat about 20-30 minutes, until syrup forms and cooks down nice and thick. Stir occasionally to prevent sticking. You might want to cut some of the larger potato pieces into smaller chunks, but leave them chunky. My mother never mashed her sweet potatoes.
Place cooked sweet potatoes into a greased casserole to fit - usually about a 2-quart dish. Top evenly with the marshmallows. They should be nice and thick and cover the potatoes well. Use more than half the bag if you need to.
Bake at 350 degrees about 20-25 minutes, until marshmallows are puffed and browned. May serve hot, or allow to cool to room temperature (that's usually how we had them, because they would have to sit out while the dressing and other dishes were in the oven).