1. Put the sugar and water in a heavy-based saucepan and cook over low heat until the sugar has dissolved.
2. Increase the heat and boil until a little of the mixture dropped in cold water separates into threads which are hard but not brittle.
3. Use a brush dipped in cold water to brush down the sides of the saucepan during cooking.
4. Gradually add the butter a little at a time, mixing well.
5. Pour the mixture into a buttered 7 x 5 inch (18 x 12.5 cm) shallow tin.
6. When almost set, cut into squares.