Arrange the sliced sweet potatoes in a lightly greased 7x11 inch casserole dish. Pour in 1/3 cup water and cover tightly with foil. Bake for 50 minutes or until tender. Carefully drain excess water out of pan.
Melt the butter in pan over medium heat and add the brown sugar, cinnamon, salt and 3 tbsp of water. Stir till smooth and bubbly, then drizzle over potatoes. Cover again with foil and cook 10 minutes. Uncover and top with marshmallows. Cook 10 more minutes. Enjoy!