1 pound (4 sticks) unsalted butter, each stick cut into 8 pieces
1/4 cup light corn syrup
2 1/2 cups sugar
1 pound bittersweet chocolate, chopped into small pieces
3 cups pecans, chopped very fine and sieved to remove fine powder
Vegetable-oil cooking spray
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.
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.
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.
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. Store in an airtight container for 3 to 4 weeks.
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