1. Place butterscotch chips and butter into a glass bowl. Heat in the microwave for about 3 minutes, stirring at 30 second intervals until smooth. Set aside to cool until firm.
2. Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan.
3. In a large bowl, beat the butterscotch chip mixture with eggs and vanilla until well blended. Combine the flour, baking powder, and salt; stir into the batter until evenly mixed. Mix in the brown sugar, chocolate chips, walnuts, and raisins. Spread the batter evenly in the prepared pan.
4. Bake for 25 minutes in the preheated oven, just until the bars begin to pull away from the sides of the pan. Cut into squares while warm.