In medium saucepan, melt butter over med. heat: add butterscotch chips and brown sugar, stirring until melted.  Remove pan from heat; add eggs; stir.  Add flour, baking powder and salt; mix thoroughly.  Stir in vanilla; set aside until cool.  When cool, stir in chocolate chips, marshmallows and nuts; spread in a greased 9 x 13 inch pan.  Bake 25 min. at 350 degrees.  Remove from oven and let cool thoroughly before cutting into squares.  These are yummy and terribly rich.  ENJOY