Line a tin or baking tray with baking parchment.
Carefully heat the sugar, butter and water, stirring constantly, in a heavy saucepan until they begin to melt.
Boil until the mixture thickens and turns light brown (for 10−15 minutes) or until it reaches 140°C.
Add the walnuts to the toffee mixture and spread out on a sheet of baking parchment.
Allow to cool and pour over the melted chocolate. Crush coarsely.
