Add to a medium saucepan the water, sugar, salt, cinnamon and cornstarch. Bring to a boil on med to med-high heat until sauce thickens. Remove from stove. Add the stick of butter and let it melt in the hot sauce. Stir to combine.
Slice the sweet potatoes (yams)into thin strips. Prepare a 9x9 square baking dish. Alternate layers of sweet potatoes and apples. Pour sauce over until well covered. It may not take all of the sauce so use your best jusgement. Cover with foil and bake at 350 for 1 hour. Enjoy!