PREP: Line a 9x9-inch (or 8x8-inch) pan with parchment paper, leaving an overhang for easy removal later. Assemble all the ingredients. In a small bowl, press the baking soda through a fine-mesh sieve to remove any lumps. Set out a heat-resistant silicone spatula and whisk near the stovetop.
PEANUTBUTTER: Add the peanut butter, butter, and salt to a microwave-safe bowl. Microwave for 20 seconds, stir, and microwave again for another 20 seconds. Continue to microwave in bursts until the peanut butter is melted. We're looking for it to become soft and somewhat pourable. Stir in the vanilla extract. Use a spatula to scrape this mixture into a very large bowl. Keep this large bowl close to the stovetop.
SUGAR MIXTURE: Combine the white sugar, corn syrup, and water in a nonstick, heavy-bottomed, medium-sized pot. Stir until the sugar is moistened with the water, being careful to not get sugar on the sides of the pot. Place the pot on the stovetop over high heat. Place the candy thermometer on the side of the pan and then cook this mixture, without stirring, until the sugar syrup reaches 285 degrees F. Watch carefully-- it can shoot up quickly!
COMBINE SUGAR MIXTURE and PEANUT BUTTER: As soon as the syrup heats 285 degrees F, remove it from the heat and immediately whisk in the baking soda. Whisk vigorously-- the syrup will grow and turn foamy. Immediately pour this mixture into the large bowl with the peanut butter mixture.
FINISHCANDY: Switch to the spatula and very quickly fold the peanut butter mixture into the foamy syrup. Work very quickly here, it will start to turn to a solid mass and look like a thick brown stringy taffy. Quickly scrape this mixture out into the prepared 9x9-inch pan and spread it with the spatula to be about an inch thick. It's okay if it doesn't entirely fill the pan. Let cool at room temperature for 3-5 hours or until completely cooled.
TOP WITH CHOCOLATE: When the candy is fully cooled, finely chop the chocolate. Add about 2/3 of the chocolate (just eyeball it) to a microwave-safe bowl. Microwave for 20 seconds, stir for 30 seconds, and repeat this process until the chocolate is melted. Once it's fully melted, add in the remaining chopped chocolate and mix until that chocolate is melted. This should bring the temperature of the chocolate to right under body temperature. (We introduce unmelted chocolate and keep stirring to cool down the chocolate; this helps it to be tempered so it won't turn streaky.)
ENJOY: 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 we don't add too much.