Put the sugar and 150 ml (1/4 pint) water in a large heavy-based saucepan, with a sugar thermometer attached, and heat gently until dissolved.

Bring to the boil, then boil until the temperature reaches the soft crack stage 132°C (270°F), when a little of the syrup dropped into cold water separates into hard but not brittle threads. Brush down the sides of the pan occasionally with a pastry brush dipped in cold water.

Add the butter a little at a time, stirring until dissolved before adding more. Pour into a greased 18 cm (7 inch) square tin. Mark into squares when almost set. When set, break along the marked lines. Store in an airtight container.