Step 1

Line a baking sheet with a silicone mat or greased piece of parchment paper; set aside. Make sure all of the ingredients are measured and ready to go.

Step 2

Place the sugar, corn syrup, and water in a medium saucepan over medium-low heat. Stir with a heatproof spatula until the sugar melts.

Step 3

Once the sugar melts, stop stirring and turn the heat up to high. Bring to a boil and continue to cook until the sugar mixture registers 265°F on a candy thermometer.

Step 4

At 265°F, quickly but carefully stir in half the pistachios, followed by the butter, cinnamon, baking soda, and salt. The mixture may bubble up a bit, so be careful.

Step 5

Stirring occasionally to keep the nuts from burning at the bottom, continue to cook until the mixture registers 305°F on the candy thermometer. Remove from the heat and stir in the vanilla.

Step 6

Immediately pour the mixture onto the prepared baking sheet and spread out with the spatula into an even layer. Sprinkle with the remaining pistachios and flaky salt. Let cool completely. Break into pieces and store in an airtight container. Place a piece of parchment paper between layers of brittle to keep from sticking together.