Lightly butter a 10- by 15-in. baking pan. In a small bowl, stir baking soda in vanilla to dissolve; set aside. In another bowl, stir together cinnamon and salt (to help the cinnamon distribute evenly when it's stirred into the sugar mixture).
In a 4- to 5-qt. saucepan over medium-low heat, use a heatproof spatula or wooden spoon to stir together sugar, 1/2 cup water, corn syrup, and 1/4 cup butter until butter is melted and sugar is completely dissolved (see âÂ€ÂœCaramelizing Sugar: Watch Closely,âÂ€Â? below). Increase heat to medium and boil sugar mixture, stirring occasionally, until it turns a deep amber and measures 335° to 340° on a candy thermometer (see âÂ€ÂœUsing a Candy Thermometer,âÂ€Â? below), 8 to 12 minutes.
Remove sugar mixture from heat and carefully stir in vanilla and cinnamon mixtures (they will bubble up). Immediately stir in pumpkin seeds and pour into prepared pan, using spatula or wooden spoon to evenly spread and fill pan. Let brittle cool at room temperature for 30 to 40 minutes.
Gently twist pan to release brittle (if necessary, run a heatproof spatula underneath brittle to help release it), then chop or break it into chunks. Store in an airtight container at room temperature for up to 2 weeks.
Caramelizing sugar: Watch closely. Sugar cooks quickly once it begins to brown, so have your ingredients measured and your tools assembled before you start. Before bringing your sugar mixture to a boil, make sure you completely dissolve the sugar and melt the butter: Stir the mixture with a flexible heatproof spatula over medium-low heat, running the spatula around the inside of the saucepan a few times to thoroughly mix everything together.
Using a candy thermometer
It can be tricky to know when a sugar mixture has reached the temperature that will yield the texture you're after. Thankfully, candy thermometers eliminate the guesswork. You'll find them at supermarkets and some hardware stores; make sure to choose one that measures temperatures up to 400°.
Test it first: If you have an old thermometer in the back of your gadget drawer, test its accuracy by immersing it in boiling water; it should read 212°. (If it doesn't, get a new thermometer.) Our favorite in the test kitchen is the Taylor Classic Candy and Deep-Fry Analog Thermometer ($13; available at housewares and home improvement stores), an inexpensive model that's easy to read and attaches securely to the pan's side.
Read it accurately: Attach the thermometer to the side of the pan after the sugar mixture has come to a boil. The placement of the heat sensor varies among brands, but generally the bottom of the thermometer must be completely submerged in the sugar mixture to get an accurate reading. If the mixture is too shallow, carefully tilt the pan until liquid completely covers the bottom of the thermometer when you want to check the temperature.
Note: Nutritional analysis is per oz.