Line a 15x10x1 inch baking pan with foil, extended over edges.
In a 3 quart saucepan melt butter. Stir in sugar, corn syrup and 1/3 cup water. Cook over medium-high heat to boiling, stirring until sugar is dissolved. Avoid splashing onto sides of pan. Clip candy thermometer to pan. Be sure bulb is well covered and not touching the bottom of the pan. Cook over medium heat, stirring frequently until thermometer registers 290 degrees (soft crack stage), about 15 minutes, the mixture will turn golden brown. Remove thermometer.
Pour into prepared pan, spread evenly. Cool 5 minutes or until top is just set.
Sprinkle chocolate pieces atop, let stand 2 minutes. Spread chocolate evenly over toffee. Top with nuts, gently pressing them into chocolate. Cool several hours or until set. If necessary, place in refrigerator.
Holding foil, lift the candy out of the pan. Break candy into pieces. To store: layer candy in an airtight container, between sheets of wax paper.