Line a 9 x 9-inch or 8 x 8-inch pan with parchment paper, leaving an overhang for easy removal. Gather all your ingredients. In a small bowl, sift the baking soda through a fine-mesh sieve to eliminate lumps. Keep a heat-resistant silicone spatula and whisk handy near the stovetop.
Add the peanut butter, butter, and salt to a microwave-safe bowl. Microwave for 20 seconds, stir, and repeat in 20-second bursts until the mixture is soft and pourable. Stir in the vanilla extract. Use a spatula to transfer this mixture to a large bowl. Keep it close to the stovetop.
Combine the white sugar, corn syrup, and water in a nonstick, heavy-bottomed pot. Stir gently until the sugar is moistened, avoiding contact with the pot's sides. Place the pot over high heat and attach a candy thermometer. Cook without stirring until the syrup reaches 285℉. Watch closely as it can heat up quickly.
As soon as the syrup heats 285℉, remove it from the heat and immediately whisk in the baking soda. Whisk vigorously and once the syrup grows and turns foamy, immediately pour the syrup into the large bowl with the peanut butter mixture.
Switch to the spatula and quickly fold the peanut butter mixture into the foamy syrup. Work swiftly, as it will begin to thicken into a solid, stringy mass. Transfer the mixture to the prepared 9 x 9-inch pan and spread it to about an inch thick. It's fine if it doesn't completely fill the pan. Allow it to cool at room temperature for 3–5 hours, or until fully set.
Once the candy is fully cooled, finely chop the chocolate. Place about 2/3 of it in a microwave-safe bowl. Microwave for 20 seconds, then stir for 30 seconds, repeating until melted. Once fully melted, add the remaining 1/3 chopped chocolate and mix until it melts. This will bring the chocolate temperature to just under body temperature, helping to temper it and prevent streakiness.
Spread the chocolate evenly across the top of the buttercrunch candy. Let buttercrunch stand at room temperature for 1 hour or until set. Remove the bars using the parchment paper overhang and cut into squares with a very sharp chef's knife. Enjoy!
Notes
Note1: If using a different brand of peanut butter, scale the salt up to 1/2 or even 3/4 teaspoon. Skippy peanut butter is pretty salty as is, so I don't add too much.Storage: The candy's shelf life depends on humidity. In high humidity, it can become sticky and may last only 1 to 2 days. In dry conditions, it can stay fresh for up to a week when stored in an airtight container at room temperature. Avoid refrigerating the candy!