Line a 9x13-inch pan with parchment, leaving overhang. Lightly grease the parchment.
Warm the sugar and corn syrup in a medium pot over medium heat, stirring, until the edges just start to bubble.
Pull off heat. Stir in peanut butter, vanilla, and fine salt until smooth and glossy.
Fold in the crispy rice cereal quickly until every piece is coated.
Press the mixture into the pan with a greased spatula. Pack gently so the bars stay tender.
Melt chocolate chips, butterscotch chips, and butter in a heatproof bowl over low heat or in 20-second microwave bursts, stirring until smooth.
Spread the melted topping over the cereal layer. Tap the pan to level the surface.
Cool until set, 45–60 minutes at room temp or 20 minutes chilled. Sprinkle flaky salt if using, then cut into bars.