How to Make It
Line a 9-inch-square pan with foil. In a medium saucepan, stir the sugar with the corn syrup and water. Add the butter and bring to a boil over high heat, stirring to dissolve the sugar. Attach a candy thermometer to the pan and cook over moderately high heat until the caramel reaches 285°, about 10 minutes.
Meanwhile, in a large heatproof, microwave-safe bowl, combine the peanut butter with the peanuts and salt. Heat the peanut butter mixture in the microwave at high power for about 11/2 minutes, until melted and hot. Stir well.
In a small bowl, whisk the vanilla with the baking soda. As soon as the caramel reaches 285°, carefully stir in the baking soda mixture; the caramel will foam and bubble up.
Immediately pour the caramel into the melted peanut butter mixture and, using a heatproof spatula, fold together as quickly as possible. You want the mixtures to be combined but not homogenized; the candy will come together very fast.
Immediately scrape the hot candy into the prepared pan and press into a flat, even layer. Let cool completely. Peel off the foil and cut the candy into squares.