Line a 13x9x2-inch baking pan with foil, extending foil over edges of pan. Sprinkle the 1/2 cup coarsely chopped nuts in pan. Set pan aside.
Butter sides of a heavy 2-quart saucepan. In saucepan melt butter. Add sugar, water, and corn syrup. Cook and stir over medium-high heat until mixture boils. Clip a candy thermometer to side of pan. Reduce heat to medium; continue boiling at a moderate, steady rate, stirring frequently, till thermometer registers 290 degree F, soft-crack stage (about 15 minutes). Watch carefully after 280 degree F to prevent scorching. Remove saucepan from heat; remove thermometer. Pour candy into the prepared pan.
Let candy stand about 5 minutes or till firm; sprinkle with chocolate. Let stand 1 to 2 minutes. When chocolate has softened, spread over candy. Sprinkle with the 1/2 cup finely chopped nuts. Chill until firm. When candy is firm, use foil to lift it out of pan; break into pieces. Store tightly covered. Makes about 1-1/2 pounds (48 servings).