1. Line a 15 x 10 inch jellyroll pan with aluminum foil.
2. Lightly coat with non-stick cooking spray.
3. Sprinkle foil with 1 cup hazelnut pieces.
4. In a large saucepan over medium high heat bring sugar, butter and water to a boil, stirring constantly.
5. Cook, still stirring, 12 minutes or until a candy thermometer registers 310 degrees-hard crack stage.
6. Pour hot mixture evenly over hazelnut pieces.
7. Let stand 20 seconds.
8. Sprinkle remaining hazelnuts on top.
9. Chill for 30 minutes in your refrigerator.
10. Turn out toffee onto a cutting board, remove foil.
11. Break into pieces with a rolling pin.
12. Store in an airtight container.
13. Cooking time includes chill time.