Set oven to “warm”, or lowest temperature.
Spray a large cookie sheet with cooking spray. Place inside warm oven.
In a heavy saucepan (2 quart), mix together sugar, corn syrup, salt, and water
Bring to boil over medium-high heat, then attach candy thermometer to saucepan. Stir often until temperature reaches 300 degrees F (150 degrees Celcius). KEEP A VERY CLOSE EYE ON IT after about 280 degrees, as the temp has a tendency to race up at that point. If you turn your back, you could burn it!
At 300 degrees, remove the pan from heat, add baking soda and butter, stir until incorporated. Quickly add in pistachios, stir until completely coated.
Remove cookie sheet from oven, and pour brittle out onto it. Use 2 buttered forks to pull the brittle mixture out from the center, till it is thinly spread and relatively even.
Cool completely, then break into pieces.