Line a 9×13-inch baking pan with parchment paper or lightly grease it.

Place the Rice Chex cereal into a large mixing bowl.

In a medium saucepan over medium heat, combine the corn syrup and sugar. Stir constantly until the sugar dissolves and the mixture just begins to simmer. Do not let it boil for long.

Remove the saucepan from heat and immediately stir in the peanut butter and vanilla until smooth.

Pour the warm peanut butter mixture over the cereal. Gently fold until every piece is evenly coated.

Transfer the mixture to the prepared pan and gently press it into an even layer using a buttered spatula or lightly greased hands.

In a microwave-safe bowl, combine the chocolate chips, butterscotch chips, and butter (if using). Heat in 20-second intervals, stirring between each, until completely smooth.

Spread the melted chocolate-butterscotch mixture evenly over the cereal layer.

Let the bars cool at room temperature for 1 to 2 hours, or refrigerate for about 30 minutes until the topping is set.

Slice into 24 squares and serve.