Spray a baking sheet with vegetable-oil cooking spray, wiping off any excess with a paper towel; set aside. In a medium heavy-bottomed saucepan, combine sugar, corn syrup, and water. Stir to combine. Cover, and bring to a boil over high heat. Cook until sugar is dissolved. Uncover, reduce heat to medium, and continue to cook without stirring until the sugar mixture reaches 295 degrees.on a candy thermometer. Remove from heat.
Stir in 3/4 cup pistachios, the butter, vanilla, and baking soda. Return to heat, and cook, stirring constantly, until the mixture reaches 300 degrees.on a candy thermometer. Remove from heat, and pour mixture onto prepared baking sheet. Working quickly, spread mixture. Set aside to cool.
Break brittle into small pieces. Place in a food processor fitted with a metal blade. Process until powdery.