Step 1: Preheat your oven to 325°F (165°C) and toast the pumpkin seeds on a baking sheet for 10-12 minutes until lightly golden and fragrant.
Step 2: Line a large baking sheet with parchment paper or a silicone baking mat and set aside.
Step 3: In a medium heavy-bottomed saucepan, combine the sugar, corn syrup, and water over medium heat, stirring until the sugar dissolves.
Step 4: Increase the heat and bring the mixture to a boil. Use a candy thermometer to monitor the temperature, cooking until it reaches 300°F (150°C), also known as the hard-crack stage.
Step 5: Immediately remove the pan from the heat and quickly stir in the butter, vanilla extract, toasted pumpkin seeds, and salt.
Step 6: Quickly add the baking soda and stir vigorously; the mixture will foam up and become airy.
Step 7: Pour the mixture onto the prepared baking sheet and spread it as thin as possible using a heat-resistant spatula.
Step 8: Allow the brittle to cool completely at room temperature for at least 30 minutes before breaking it into pieces.