Nonstick vegetable oil spray
1 Cup sugar
1/2 Cup light corn syrup
1 Cup unsalted, shelled raw natural pistachios, very coarsely chopped
1 Tbsp unsalted butter
1 tsp Kosher Salt
3/4 tsp baking soda
Fleur de Sel, or Coarse Grey Sea Salt
Line a rimmed baking sheet with parchment paper. Spray parchment with nonstick spray and set aside.
Whisk sugar, corn syrup, and 3 Tbsp water in a medium saucepan. Stir over medium heat until sugar dissolves. Clip a candy thermometer to the saucepan, bring sugar-water mixture to a boil, and when thermometer registers 290°F, cook for 3-4 min. Using a heat-proof wooden or silicone spatula, stir in chopped pistachios, butter, and kosher salt (syrup will seize up initially, but will melt again as mixture heats through).
Continue to cook candy syrup, stirring often, until thermometer registers 300°F and pistachios are golden brown, 3-4 minutes. Caramel should be pale brown (this will darken slightly as it cools). Sprinkle baking soda over the molten brittle, and stir quickly to blend caramel thoroughly (mixture will bubble vigorously with the addition of baking soda).
Immediately pour caramel onto the prepared baking sheet and, using the heat-proof spatula, quickly spread out brittle as thinly as possible. Sprinkle sea salt over the hardening candy, and allow caramel to cool completely.
Break brittle into pieces.
DO AHEAD: Brittle can be made up to 1 week ahead. Store at room temperature, in an airtight, sealable container, between sheets of parchment paper (to prevent sticking).