Line an 8x8-inch pan with aluminum foil and grease with butter. Set aside.

Combine sugar, corn syrup, water, and salt in a medium saucepan over medium-high heat, stirring until sugar is dissolved.

Insert a candy thermometer and cook, without stirring, until the temperature is 300°F. Immediately remove from heat and stir in peanut butter.

Scrape into prepared pan, press smooth with a spatula, and sprinkle with kosher salt.

Using a knife, score into inch-size squares while the candy is still warm. Let cool completely.

Remove from pan and slice into pieces along the score marks.

Dip each piece in chocolate and place on waxed paper to harden.