Cook the 6 medium size sweet potatoes in their jackets until tender. Remove skins. Meanwhile combine remaining ingredients in a saucepan and cook stirring until thickened. Arrange sweet potatoes, either whole or cut in lengthwise slices in a baking dish. Pour sauce over them. Cover and bake in a moderate (350 degree) oven for 20 minutes. Uncover and bake 15 minutes longer. Cut up pieces of marshmallow may be sprinkled over the potatoes during the last 15 minutes of baking.