Spicy Pumpkin Seed Brittle

Heat oven to 350 ºF. Scatter pumpkin seeds in an even layer on a rimmed baking sheet; place in oven for 8 to 10 minutes, tossing once or twice, until lightly toasted and fragrant. Line a large baking sheet with 2 sheets of nonstick foil. In a small cup, whisk baking soda, cinnamon, salt and cayenne, if using, and set aside. In a large saucepan, mix sugar, syrup and water. Cover and bring mixture to a boil over high heat. Add butter; stir until melted. Attach a candy thermometer to the side of the pan with the tip in the sugar mixture. Over medium-high heat, cook, without stirring, 20 minutes or until thermometer registers 305 ºF. Stir in the seeds and vanilla. Remove from heat. Sprinkle the baking soda mixture over top; stir vigorously with a heat-resistant rubber spatula 15 seconds, being careful not to spatter. Immediately pour onto prepared baking sheet; spread with a metal offset spatula as thinly as possible. Let cool. Break into pieces.