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 1 ½ 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 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.
Melt the chocolate over a double boiler at low heat. Using two forks, dip the candy and coat well in the chocolate. Place on a cookie sheet lined with parchment paper to set.