1. Grease and line an 18 cm square baking tin with greaseproof paper.
2. Place the sugar in a saucepan set over a medium heat.
3. Leave the sugar to dissolve undisturbed until it starts to change color, swirling it gently until it starts to turn amber.
4. Cook the sugar until it registers 138°C on a sugar thermometer. Add the peanuts and swirl to coat.
5. Continue to cook until the sugar reaches 149°C. Pour into the prepared tin and leave to cool and set for 2 hours.
6. Turn out and break into pieces before serving.