Preheat the oven to 350 F and grease and line an 8x8-inch square baking pan.
In a large mixing bowl, whisk together the melted butter, brown sugar, eggs, and vanilla extract until smooth.
Sift in the flour, baking powder, and salt, and gently fold together until just combined.
Add the butterscotch chips and fold them through.
Transfer the mixture into your prepared baking pan, spreading it into an even layer.
Bake for 35 to 45 minutes or until a skewer poked into the center of the brownie comes out with just a few moist crumbs, but no wet batter. If they start to become too browned on the top, loosely cover the pan with foil.
Let the brownies cool in the pan for about 15 minutes before transferring them to a wire rack to cool completely.