Reduce heat to low; cover and cook until thickened and dark brown, 9-11 hours, stirring occasionally (stir more frequently as it thickens to prevent sticking).
Uncover and cook on low, 1 hour longer. If desired, stir with a wire whisk until smooth. Spoon into freezer containers, leaving 1/2-in. headspace. Cover and refrigerate or freeze.
Nutrition Facts
2 tablespoons: 68 calories, 0 fat (0 saturated fat), 0 cholesterol, 9mg sodium, 17g carbohydrate (16g sugars, 1g fiber), 0 protein.