Lightly coat a jellyroll pan or large cookie sheet with cooking spray; set aside.
Combine brown sugar and next 4 ingredients in a heavy 2-quart nonstick saucepan. Bring to a boil over medium-high heat, stirring constantly with a wooden spoon. Cover and cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan.
Uncover, and attach a candy thermometer to side of pan. Cook, stirring constantly, until thermometer registers 280°. Add butter.
Cook, stirring constantly, until thermometer registers 300° (hard crack stage). Remove thermometer. Quickly stir in pistachios, immediately pouring mixture onto prepared pan. Quickly press nuts into a single layer within candy mixture. (Candy will not cover pan.) Cool completely; break brittle into pieces. Store in an airtight container.