Preheat oven to 350 degrees F. Grease a 15x10x1-inch baking pan; set aside.
In a large bowl, combine cake mix, brown sugar, melted butter, and egg. Beat with an electric mixer on medium speed for 1 minute or until combined, scraping bowl occasionally. Stir in the butterscotch chips.
Pat dough into prepared pan. Bake in the preheated oven for 15 to 20 minutes or until a toothpick inserted in center comes out clean. Cool completely in pan on a wire rack.
Core and thinly slice the apple, then saute the slices in the butter, brown sugar, and cinnamon until they are nice and soft.
Create a simple butterscotch sauce by microwaving butterscotch chips and milk for 45-55 seconds. Whisk the mixture together until it is smooth and easily poured.
Cut into bars. Spoon apple slices onto each one and drizzle sauce across it. Makes 16 bars.