1. Combine ingredients in a saucepan and set over medium heat. Stir constantly until the mixture is about to boil and all the sugar is dissolved.

2. Cool to room temperature, pour into a shallow pan (an 8-inch square cake tin is ideal), and freeze.

3. The mixture will take from 3 to 6 hours to freeze and, because of its relatively low sugar content, will be very solid. To serve, set in refrigerator for 30 minutes to temper the texture slightly.