1. Coat a 13-by-17-inch rimmed baking sheet with cooking spray. Sift pistachios using a fine sieve to remove dust and fine pieces. Heat sugar and water in a heavy saucepan over medium-high heat, stirring occasionally with a wooden spoon and washing sides of pan often with a wet pastry brush to prevent crystals from forming, until dark golden, about 9 minutes.

2. Stir in pistachios, and immediately pour mixture onto baking sheet. Distribute pistachios evenly using a rubber spatula. Let stand until hardened, about 15 minutes.

3. Lightly bang baking sheet on a counter to break brittle into pieces. Serve with ice cream.