Heat oven to 350 degrees. Grease a 13 x 9 baking dish with cooking spray.
In a sauce pan over medium heat, melt the butterscotch chips and butter, stirring constantly until smooth (this can also be done in the microwave). When melted remove from heat and transfer the contents of the pan to a mixing bowl. Don't over heat the butter and butterscotch chips or they will separate, so make sure you watch them carefully and don't cook them too long.
Cool the butterscotch and butter mixture to room temperature. Mix in the flour, brown sugar, baking powder, salt, vanilla, and eggs into the butter and butterscotch mixture. Make sure you blend everything together really well. The batter will be thick. Mix in the chocolate chips and marshmallows. Spread the blondie batter into the pan.
Bake for 25-30 minutes. The center will feel a little soft, but will firm while cooling. Let cool before cutting.