1. In a large heavy bottomed saucepan, combine the butter, sugar and salt. Cook over medium heat, stirring until the butter is melted. Allow to come to a boil, and cook until the mixture becomes a dark amber color, and the temperature has reached 285 degrees F
(137 degrees C). Stir occasionally.
2. While the toffee is cooking, cover a large baking sheet with aluminum foil or parchment paper.
3. As soon as the toffee reaches the proper temperature, pour it out onto the prepared baking sheet. Sprinkle the chocolate over the top, and let it set for a minute or two to soften. Spread the chocolate into a thin even layer once it is melted. Sprinkle the nuts over the chocolate, and press in slightly. Putting a plastic bag over your hands will minimize the mess.
4. Place the toffee in the refrigerator to chill until set. Break into pieces, and store in an airtight container.