This recipe was adapted and updated from Ann Edie's Disappearing Marshmallow Brownies published in First Church Fare compiled by First Presbyterian Church, Bel Air, Maryland 1990
Preheat the oven to 350. Butter 8x8-inch baking dish and set aside.
In small pan, melt together the butterscotch chips and the butter, stirring constantly, until melted. Pour into a large bowl. Allow to cool to close to room temperature (lukewarm) then stir in the flour, brown sugar, baking powder, salt, vanilla, and egg (make sure the egg is added last to avoid it being heated by the butterscotch mixture) until well mixed and uniform. Fold in the marshmallows and chips.
Scrape into the prepared pan and bake for 25 minutes or until most of the pan looks dry and golden and the absolute center area looks just slightly underbaked.
Cool completely, in pan, on a wire rack.
Slice and serve.
Notes
Resist the urge to cut them warm! They really need to set up during the cooling process in order to make a sliceable bar.