Spray a parchment-lined bakingsheet with nonstick spray; setaside. Bring sugar, corn syrup,and 3 tablespoons water to a boil ina medium saucepan over mediumheat, stirring to dissolvesugar. Fit saucepan withthermometer and cook untilthermometer registers 290°F,3–4 minutes.
Stir in pumpkin seeds, butter,and kosher salt and cook,stirring often, until pale brownand thermometer registers305°F, 3–4 minutes.
Stir in baking soda andcinnamon (mixture will bubblevigorously), then immediatelypour caramel onto preparedsheet. Using a heatproofspatula, quickly spread out andsprinkle with sea salt; let cool.Break brittle into pieces.
DO AHEAD: Brittle canbe made 1 week ahead. Storeairtight, layered betweensheets of parchment paper,at room temperature.