Line a 15- x 10-inch jellyroll pan with heavy-duty aluminum foil; lightly grease foil. Sprinkle with 1 cup pecans to within 1 inch of edges.
Bring sugar, butter, and 1/3 cup water to a boil in a heavy saucepan over medium heat, stirring constantly. Cook over medium-high heat, stirring constantly, 12 minutes or until a candy thermometer registers 310° (hard crack stage). Pour over pecans; sprinkle with chocolate pieces. Let stand 30 seconds.
Sprinkle with remaining 1/2 cup pecans. Chill 30 minutes. Break up toffee using a mallet or rolling pin. Store in an airtight container.