Line a 9x13-inch baking dish with parchment paper, allowing some overhang for easy removal.
In a large mixing bowl, combine 1-1/2 cups peanut butter, 1 tablespoon unsalted butter, 1/4 teaspoon salt, and 1 teaspoon vanilla extract. Mix until creamy and well combined.
In a medium saucepan, combine 1 cup sugar, 3/4 cup light corn syrup, and 2 tablespoons water. Heat over medium heat, stirring constantly until sugar dissolves and mixture boils.
Remove saucepan from heat and quickly stir in 1/2 teaspoon baking soda; mixture will bubble up.
Pour hot sugar mixture into peanut butter mixture. Stir well until smooth and creamy.
Fold in 3 cups of your favorite crispy cereal (like Rice Krispies or Cornflakes) until all cereal is coated.
Transfer mixture into prepared baking dish, spreading and pressing down firmly to create a compact layer.
Melt 3.5-ounce bar of dark chocolate with 1 teaspoon coconut oil or shortening in a double boiler or microwave-safe bowl until smooth.
Pour melted chocolate over peanut butter crunch mixture, spreading evenly with a spatula. Let chocolate set at room temperature or refrigerate to firm faster.
Once chocolate has set, lift candy out using parchment paper overhang, cut into squares or bars, and serve.