Line a 13x9 pan with foil and generously grease with butter. Set aside.
In a large pot, combine corn syrup, sugar, and butter over medium-low heat, stirring occasionally until butter is melted and ingredients are well-combined.
Increase heat to medium-high and stir frequently. Once mixture just begins to boil (as soon as it starts bubbling) remove from heat and stir in peanut butter, vanilla extract, and salt.
Stir well until mixture is smooth.
Stir in cereal -- make sure ingredients are well-combined.
Spread ingredients evenly into prepared pan, Press down to pack into pan (I like to place a sheet of wax paper over the mixture and use my hands to press the scotcharoos down and make them even -- be careful as it may be hot), then discard wax paper.
Set aside and prepare your chocolate topping.
In a medium-sized microwave safe bowl, combine semisweet chocolate chips and 1 cup (170g) butterscotch chips.
Microwave for 30 seconds, stir well, and return to microwave at 15-second increments, stirring well in between until mixture is completely melted. Set aside.
Immediately melt remaining ¼ cup (42g) butterscotch chips (15 second increments, stirring between).
Spread chocolate butter-scotch mixture evenly over prepared scotcharoos. Drizzle melted butterscotch chips over the chocolate and then drag a toothpick through the chocolate for swirls.
Allow chocolate to set (about an hour at room temperature) before cutting and serving.