In a large heavy bottomed saucepan, combine the butter, sugar and salt. Cook over medium heat, stirring until the butter has melted. Allow to come to the boil, and cook until the mixture becomes a dark amber colour, or the temperature has reached 140 degrees C. Stir occasionally. Handle very carefully.
While the toffee is cooking, cover a Swiss roll tray with foil or greaseproof paper.
As soon as the toffee is cooked, (handle it very carefully as it will be very hot) pour into the baking tray.
Melt the chocolate and spread a thin layer over the toffee. Sprinkle the nuts over the chocolate, and press in slightly.
Place the toffee in the fridge until it sets. Break into pieces, and store in an airtight container.