Combine all the ingredients in a heavy-bottomed saucepan and cook over medium heat, stirring constantly and occasionally washing down the sides of the pan with a wet pastry brush, until the mixture reaches 293°F/145°C.
Pour the toffee onto a silicone baking mat 12 by 16 in/30 by 41 cm; it should be about ⅛ in/0.3 cm thick. Cool slightly until the toffee begins