In a saucepan over low heat, melt butterscotch chips and butter; cool for 10 minutes. In a mixing bowl, beat eggs, brown sugar and vanilla. Add butterscotch mixture; mix well. Combine flour, baking powder and salt; add to batter and mix well. Stir in marshmallows, chocolate chips and nuts. Spread into a greased 13 inch x 9 inch x 2 inch baking pan. Bake at 325? for 25-30 minutes or until brownies test done with a toothpick. Cool before cutting.