Line a 9×13 pan with parchment paper or foil that has been sprayed with nonstick spray. Preheat oven to 350 degrees.
In medium sauce pan melt butter, butterscotch chips and brown sugar over medium low heat. Stir until melted (this may look a little strange but just keep stirring until it comes together). Remove from heat and add the eggs and vanilla stir.
Next add the flour, baking powder, and salt. Mix well and then set aside to cool slightly. Once the mixture is cool, stir in chocolate chips, marshmallows and nuts (if using).
Spread into prepared pan and bake 22 to 25 minutes, until golden brown. Allow to cool completely before cutting into bars.