Nonstick vegetable oil spray
1 cup sugar
½ cup light corn syrup
1 cup raw shelled pumpkin seeds (pepitas)
2 tablespoons unsalted butter
1 teaspoon kosher salt
¾ teaspoon baking soda
⅛ teaspoon ground cinnamon
¼ teaspoon flaky sea salt
A candy thermometer
Spray a parchment-lined baking sheet with nonstick spray; set aside. Bring sugar, corn syrup, and 3 Tbsp. water to a boil in a medium saucepan over medium heat, stirring to dissolve sugar. Fit saucepan with thermometer and cook until thermometer registers 290°, 3–4 minutes.
Stir in pumpkin seeds, butter, and kosher salt and cook, stirring often, until pale brown and thermometer registers 305°, 3–4 minutes.
Stir in baking soda and cinnamon (mixture will bubble vigorously), then immediately pour caramel onto prepared sheet. Using a heatproof spatula, quickly spread out and sprinkle with sea salt; let cool. Break brittle into pieces.
Do Ahead: Brittle can be made 1 week ahead. Store airtight, layered between sheets of parchment paper, at room temperature.