Preheat oven to 350 degrees. Lightly spray a 9 x 9 inch square pan with non-stick cooking spray and set aside. In large bowl, put the margarine and white sugar and cream together with a large spoon. In a small bowl, melt the two squares of the semi-sweet baking chocolate bar in the microwave. Pour the melted chocolate in with the margarine and white sugar and then add the eggs. Mix well and add the flour, baking powder and salt. Combine well and then mix in the milk chocolate chips, the semi-sweet chocolate chips and the peanut butter chips with a large spoon. Pour into the prepared pan and bake for 30-35 minutes or until a toothpick inserted into the middle of the brownies comes out clean. Remove pan to wire rack to cool. Serve with a little powdered sugar on top, leave plain or add your favorite frosting. Store covered at room temperature. Makes 12 servings.