Heat the oven to 350°F (175°C). Grease an 8-inch square baking pan, then line it with parchment paper, leaving an overhang on two sides so the cooled blondies can be lifted out easily.

In one mixing bowl, whisk the melted and slightly cooled butter with the firmly packed brown sugar until smooth and evenly combined.

Whisk in the egg and vanilla extract until the mixture is glossy and well blended.

Add the flour, baking powder, and fine salt. Stir just until no dry streaks remain, taking care not to overmix the batter.

Fold in the butterscotch baking chips until they are distributed throughout the thick batter.

Spread the batter evenly in the prepared pan. A lightly greased spatula makes this thick mixture much easier to smooth into the corners.

Bake for 20 to 25 minutes, until the edges are set and lightly browned while the center remains slightly soft. A toothpick inserted into the middle should come out with a few moist crumbs rather than wet batter.

Cool the blondies completely in the pan for 30 to 60 minutes before cutting them into 16 squares. For especially clean cuts and a denser crumb, chill them briefly after they have cooled.