In a 2-quart heavy saucepan melt butter over low heat. Stir in sugar, water, and corn syrup. Bring to boiling over medium-high heat, stirring until sugar is dissolved. Avoid splashing side of saucepan. Clip a candy thermometer to side of pan. Cook over medium heat, stirring frequently, until thermometer registers 290 degrees F, soft-crack stage (about 12 minutes). Mixture should boil at a moderate, steady rate with bubbles over entire surface. (Adjust heat as necessary to maintain a steady boil and watch temperature carefully during the last 5 minutes of cooking as temperature can increase quickly at the end.) Remove from heat; remove thermometer.
Carefully pour corn syrup mixture into prepared pan; spread evenly. Cool 4 to 5 minutes or until top is just set. Sprinkle evenly with chocolate pieces; let stand for 2 minutes. Spread softened chocolate into an even layer over toffee layer. Sprinkle with nuts; lightly press into chocolate. Let stand at room temperature several hours or until chocolate is set. Use foil to lift candy out of pan; break into pieces. Store with waxed paper between the layers in an airtight container at room temperature for up to 2 weeks.