Grease or spray two nine- by 13 inch aluminum pans or baking sheets with nonstick cooking spray.
In a heavy metal saucepan (four to five quarts), melt together the baking sticks, sugar, and salt, whisking until smooth. Over medium-high heat, bring the mixture to a boil, stirring occasionally for about 10 minutes and then constantly for five or more minutes, until the mixture is a deep golden color (300 degrees Fahrenheit on a candy thermometer). Stir thoroughly to prevent the mixture from burning and watch the mixture carefully during the last few minutes.
Immediately stir in two cups chopped nuts, then carefully pour the hot toffee into the center of the prepared aluminum pans or baking sheets. With a silicone or metal spatula, smooth the toffee flat and thin. Let cool for a minute or two.
Sprinkle the chopped chocolate over the surface of the toffee. When the chocolate has melted (about four to five minutes), spread it evenly across the toffee with the spatula.
Place the tray of toffee in the freezer and freeze until the chocolate layer is firm, about 30 minutes. Remove from freezer and break toffee into pieces. Store in an airtight container. The toffee will keep for several weeks in the refrigerator.