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.
In a medium-sized microwave safe bowl, combine semisweet chocolate chips, 1 cup (170g) butterscotch chips, and 1 teaspoon coconut oil or vegetable shortening, if using.
Microwave for 30 seconds, stir well, and return to microwave at 15-second increments, stirring well in between until mixture is completely melted and smooth.
Immediately melt remaining ¼ cup (56g) butterscotch chips and ½ teaspoon coconut oil or vegetable shortening (heat in 15 second increments, stirring between).