Line a 9-inch square baking pan with aluminum foil. In a sturdy medium saucepan, combine the sugar, corn syrup, water and butter. Over a high-heat bring the mixture to a boil and stir until the sugar is dissolved. Attach the candy thermometer to the side of the pan. Continue to cook the mixture over medium-high heat until the caramel reaches 285 degrees F, which will take about 10 minutes.

Combine the peanut butter, peanuts and salt in a large microwave safe bowl. Heat the peanut butter mixture in the microwave until it's hot, and mix it well.

In a small bowl, combine the vanilla and baking soda. As soon as the caramel reaches 285 degrees F, very carefully add the vanilla/baking soda mixture. It will cause the caramel to bubble up and foam. This aerates the caramel. Immediately pour the caramel over the peanut butter mixture and stir well using a heatproof spatula. Work quickly as the candy comes together fast. Pour the hot candy into the foil-lined baking pan, and press into a flat even layer, pushing the candy into the corners. Let the candy cool completely. Peel off the foil and cut the candy into squares. Store the candy in an airtight container for up to 2 weeks.