Put the pumpkin seeds onto a baking sheet and place it in the top half of the oven for 20 to 25 minutes, or until the seeds start to brown slightly. Remove and set aside.
Grease a second baking sheet with a small amount of butter, or spray with nonstick spray, and set aside.
Combine the sugar, corn syrup and water in a large saucepan and place over medium-high heat. Bring to a boil and stir with a wooden spoon until the sugar dissolves. Clip the candy thermometer to the side of the saucepan and let the mixture boil until the thermometer reads 260 degrees (the hard-ball stage), about 10 minutes. Add the roasted pumpkin seeds, chili powder and cayenne and stir well. Let the thermometer come up to 300 degrees F (the hard-crack stage), and then remove the pan from the heat, stir in the vanilla and pour the brittle onto the greased baking sheet. Spread out with the wooden spoon and set aside to cool. When completely cool, break the brittle into small pieces. Store in an airtight container with waxed paper between the layers.