Preheat oven to 325 degrees F (165 degrees C). Grease a 9x13-inch baking pan with cooking spray.
Stir cake mix and brown sugar together in a large bowl. Mix in milk and egg until well blended. Fold in butterscotch chips, chocolate chips, and walnuts. Spread dough evenly in the prepared baking pan.
Bake in the preheated oven until edges start to pull away from the sides of the pan, 25 to 30 minutes.