1. Spray a 15-by-10-inch baking pan, a 16 1/2-by-11 1/2-inch baking pan, and an 8-inch square baking pan with vegetable-oil spray. In a heavy 3-quart saucepan, combine butter, 1/2 cup water, corn syrup, and sugar. Clip on a candy thermometer. Bring to a boil over high heat, stirring with a wooden spoon. Continue stirring until mixture thickens, about 2 minutes. Wash down sides of pan with a pastry brush dipped in water to remove sugar crystals. Reduce heat to low; stop stirring. Let mixture come to a boil.

2. Let boil, without stirring, until temperature reaches 280 degrees.(soft-crack stage). This will take from 35 minutes to just over an hour; it is essential that the mixture continues to boil. Remove from heat. Without scraping pot, pour into prepared pans as evenly as possible. If needed, use a spatula to smooth. Let cool at room temperature for 1 hour.

3. After 45 minutes of cooling, melt chocolate in a double boiler over medium-low heat, stirring with a rubber spatula. Pour over toffee; spread with a spatula if necessary. Let cool about 15 minutes. Sprinkle with nuts; press them into the chocolate.

4. Let stand at room temperature for 24 hours. Using a large knife, lightly score 1 3/4-by-2 3/4-inch rectangles over chocolate. Cut toffee along scored lines; lift pieces out with a spatula. Alternatively, toffee may be broken into shards.