If using fresh sweet potatoes, place in a kettle; cover with water and cook, covered for 25-35 minutes or just until tender. Drain; cool slightly. Peel and cut into chunks. Place cooked or canned sweet potatoes in a 2 litre baking dish. In a small saucepan, combine butter, syrup, brown sugar and cinnamon; cook and stir until mixture boils. Pour over potatoes. Bake at 180ºC for 30-40 minutes or until heated through.