Line a 9 × 13-inch baking pan with parchment paper or lightly grease it. Set aside.
Place the Rice Chex cereal in a large mixing bowl.
In a medium saucepan over medium heat, combine the light corn syrup and granulated sugar. Stir constantly until the sugar dissolves and the mixture just begins to simmer. Avoid boiling for too long.
Remove the saucepan from the heat. Immediately stir in the creamy peanut butter and vanilla extract until the mixture becomes smooth and well combined.
Pour the warm peanut butter mixture over the Rice Chex cereal. Gently fold until all the cereal pieces are evenly coated.
Spoon the cereal mixture into the prepared baking pan. Using a buttered spatula or lightly greased hands, gently press it into an even layer without crushing the cereal.
In a microwave-safe bowl, combine the semi-sweet chocolate chips, butterscotch chips, and optional butter. Microwave in 20-second intervals, stirring between each interval until completely melted and smooth.
Spread the melted chocolate and butterscotch mixture evenly over the cereal layer.
Allow the bars to cool at room temperature for 1–2 hours, or refrigerate for approximately 30 minutes until the topping is firm.
