Line an 8 inch square pan with nonstick foil (or just grease it) and preheat the oven to 350 degrees F. You can use glass or metal.
Put the butter in a microwave safe bowl and heat for 30 seconds. Add the morsels and stir to coat with butter. Heat for another 30 seconds and stir. Repeat until morsels are melted. Let cool.
In a mixing bowl, stir together the brown sugar, egg and vanilla. Mix in the melted and cooled butterscotch mixture, then add the baking powder, salt and flour. Stir until smooth, then stir in the chocolate chips, marshmallows and pecans.
Bake at 350 for 20 to 25 minutes. Let cool, then cut into large squares. If you have the time, I recommend cooling completely and chilling before slicing.
Notes
These brownies may sound super sweet, and but they're just the right level of sweetness if you add the nuts. The nuts offset the sugar, so if you don't like nuts you might want to go with a different blondie recipe. Simply lowering the sugar will change the texture of the brownie and may make it a little cake-y.