Pat Out Bars. Transfer the cereal mixture to your prepared pan and gently pat it into an even layer with a flexible spatula or your fingers. (Try not to overly compact this mixture since you want it to have soft scotcheroos!
Make The Topping. In a microwave-safe bowl, combine chocolate chips, butterscotch chips, and butter. Melt in the microwave on low power in 10-15 second intervals, stirring in between each interval, until the mixture is smooth and even. It will be thick! (Alternatively, you can melt this mixture in a small saucepan on the stovetop over low heat.)
Layer. Spread the melted chocolate mixture over the cereal layer and use an offset spatula to smooth the chocolate into an even layer.
Set, Serve & Store. Let the chocolate layer set 1-2 hours before cutting into bars, for best results. (You can speed this process up by covering the pan and refrigerating for 45-60 minutes.) Garnish with flaky sea salt, if desired, and cut into squares or bars. These scotcheroos are rich, so I prefer cutting them into 12-18 smaller bars. Transfer leftover scotcheroos to an airtight container and store at room temperature 1-3 days.