In a large heavy saucepan, melt butterscotch chips with butter. Stir in flour, brown sugar, baking powder, salt, vanilla, and egg. Fold in pecans, marshmallows, and chocolate chips. Spread batter into a greased and floured 9-inch square baking pan. Bake at 350° for 20 to 25 minutes. Do not overbake - center of brownies will still be soft and a little jiggly.