Wash the pumpkin seeds well and remove any bits of pumpkin pulp.

Preheat the oven to 325 degrees.

Lay the seeds out on an unlined, ungreased metal baking sheet and let them dry while the oven is heating. Once the oven is hot place the pumpkin seeds inside and bake for 30 minutes.

After 30 minutes stir the seeds around and then continue to bake until seeds start to brown slightly and begin to have an aroma, 20 to 30 minutes. Once the seeds are cooked, remove from the oven.

Butter a large metal baking sheet and set aside.

Measure out the baking soda, vanilla, and butter into a small bowl and set it aside.

In a medium saucepan, bring the water, corn syrup, and sugar to a boil over medium high heat. Stir the mixture while the sugar is dissolving. Once the sugar is dissolved and the mixture begins to bubble, don't stir anymore.

Using a candy thermometer, allow the mixture to reach 285 degrees (soft crack stage). Stir in the toasted pumpkin seeds and continue cooking. Cook until the mixture reaches 300 degrees (hard crack stage), and be sure to stir often once the seeds have been added to prevent them from sticking to the bottom of the pan.

Remove the sugar and seed mixture from the heat and stir in the baking soda, vanilla, and butter. The mixture will puff up and foam; that's normal. Stir until it almost stops foaming and then quickly pour the brittle, as thin as possible, onto the buttered baking sheet.

You can pick up the edges of the pan and tilt it back and forth to help the brittle spread, just don't spread it with a spoon. While the brittle is still hot, sprinkle on the kosher salt.

Let the brittle completely cool and then break it into pieces. Store in an airtight container or a plastic freezer bag at room temperature for up to two weeks.