Line an 8x8-inch pan with parchment paper and set aside.
In a small saucepan over medium heat, stir together peanut butter and honey until smooth and just starting to bubble.
Remove from heat and stir in vanilla extract and salt, if using.
Quickly mix in the crispy rice cereal until well coated.
Transfer the mixture into the prepared pan and press it down firmly with a spatula or your hands.
Refrigerate for at least 1 hour or until firm, then slice into bars.