Preheat your oven to 350°F. Lightly spray the bottom and sides of a 9'' x 13'' pan with non-stick spray and set aside.
In a large saucepan over medium-low heat, melt the butter and butterscotch chips, stirring constantly. Do not let the mixture darken.
Remove from heat – let cool until mixture is only lukewarm.
In a medium bowl, whisk together flour, brown sugar, baking powder, and salt.
Once the butterscotch mixture is the slightest bit warm, add vanilla and eggs, and stir well to combine.
Then add the flour mixture and stir to combine.
Quickly and gently fold in marshmallows and chocolate chips, until just combined. Do not over stir.
Spread mixture into prepared pan and bake for about 20 minutes or so, taking care to not over bake.
Let cool COMPLETELY before cutting into individual bars.
