Step 1

Preheat oven to 350º F and lightly grease a 9x13-inch baking dish with butter or non-stick spray.

Step 2

In a large bowl, beat together cake mix, canola oil, eggs and vanilla extract, mixing until mixture just comes together.

Step 3

Divide batter in half and press one half into prepared baking dish to form an even layer.

Step 4

In a medium, heat-proof bowl, butter, chocolate chips and butterscotch chips, then microwave in 30-second intervals, stirring in between, until mixture is fully melted and smooth.

Step 5

Stir condensed milk and salt, if using, into chocolate chip mixture until completely incorporated, then pour over base cake layer.

Step 6

Drop spoonfuls of remaining batter over chocolate chip middle layer, then place baking dish in oven.

Step 7

Bake for 20-25 minutes, or until top is golden brown, then remove from oven and let cool.

Step 8

Once cool, slice into bars, serve and enjoy!