Preheat the oven to 350°F. Lightly coat a 9×13-inch baking dish with nonstick cooking spray.
In a large mixing bowl, stir together the yellow cake mix, vegetable oil, and eggs until a thick dough forms.
Fold in the butterscotch chips and chocolate chips until evenly distributed.
Press the dough evenly into the prepared baking dish. Because the dough is thick, use a spatula or lightly greased hands to spread it into an even layer.
Bake for 20–25 minutes, or until the edges are lightly golden and the center is set.
Let the bars cool in the pan for 10–15 minutes before cutting into 12 squares.
Serve warm or at room temperature.
Notes
Keep the leftovers in an air tight container for up to 5 days.